• Practice problems.
• https://leetcode.com/problems/minimum-path-sum. Did BFS first. Works, but not as fast. Wrote a secondary recursive solution, with both backtracking and DP. Faster.
• https://leetcode.com/problems/4sum. Wrote a beautiful generic solution to the ksum problem. Recurse down to the 1sum case, not the 2sum! The exit condition is just the target being in the remaining nums!
• https://leetcode.com/problems/3sum-closest. Similar.
• https://leetcode.com/problems/next-permutation/. Dumb question. Too specific for an interview, and doesn't show much problemsolving skill.
• https://leetcode.com/problems/swap-nodes-in-pairs/. Recursive linked list swapping. Just define to a temp variable, perform the swap, the recurse to the next one. Could totally be solved iteratively.
• https://leetcode.com/problems/reverse-nodes-in-k-group/. Same thing, but instead of k=2 (swap every pair), swap every k. If you're swapping 2 nodes, each node needs 2 connections for each neighbor, so 4 connections total. Since the root head has no left, you're doing 3 connections on each iteration/recursion.
• https://leetcode.com/problems/sudoku-solver/. A literal solver for a given sudoku board. Just recurses, populating the board with a new (valid) number and passing it through, then exiting in the base case where there are no valid plays. Backtracking. Not DP. Takes about 40 lines.
• Remember {} is the set literal in python.
• Jack borrowed the BMW for a 7am ride with Teej.
• 
• Run, pull, sauna, meditate, stretch.
• Watched the truth about emanuel, the 2013 movie which servant allegedly ripped off. It was decent until the ending. The similarity is the fake baby, but not much else. Overall I think Servant is distinct enough to survive.
• UFC 246, mcgregor cerrone.
• Got kabuli naan, my favorite.
• Dimensional is the company that Rob partners with for data / portfolio suggestions (regarding my folks' retirement).

• Remember the tool pycallgraph, useful in tandem with cprofile for getting lots of profile information about your module.
• NBC (Comcast) added another streaming service, Peacock.
• Only 5 companies have ever hit the trillion cap. Four comma club!
• Even in a workplace environment, put yourself in your colleagues' shoes. See things from your customer's perspective. Really helps.
• Placed Amazon fresh order last night.
• Finished servant season 1. Good, but left lots of questions and loose ends. Need season 2.
• New Eminem album.
• I love the integrations between the google calendar and gmail. Autocreation of flights, hotels, events, etc - fantastic.
• Really interesting:
• 
• Got an unsolicited call from facebook today. Recruiter for senior positions in infra/scalability. Bay area based. Set up a coding interview for next week.
• Practice problems.
• https://leetcode.com/problems/combination-sum-iii. Backtracking. Define the recursive case, define some fail-fast mechanisms as exit conditions. Throw loops in if you need to check multiple starting points.
• https://leetcode.com/problems/climbing-stairs. Recursion, with dp for speed.
• https://leetcode.com/problems/plus-one. Reverse to LSB first, the iterate toward more significant digits, incrementing or carrying the one.
• https://leetcode.com/problems/merge-sorted-array/.
• https://leetcode.com/problems/subsets. Generate all subsets. Just iterate, and add the current number to all previous solutions. Exponential.
• https://leetcode.com/problems/word-search/. Seeing if a word exists in a 2d grid. Cool problem. I love graphs.
• https://leetcode.com/problems/unique-paths/. Recursion with DP and backtracking. Sum left and up. Just think about the recursive case, you'll get it!
• https://leetcode.com/problems/unique-paths-ii. Same, but with obstacles. A bit harder. Lots more corner cases to consider.
• https://leetcode.com/problems/sqrtx. Iterative, or binary search across range(0, maxint).
• https://leetcode.com/problems/count-and-say. Bottom-up iteration!
• https://leetcode.com/problems/valid-sudoku/. Writing a sudoku validator. Not hard.
• https://leetcode.com/problems/first-missing-positive/. Didn't solve this one, looked it up though. Very complex problem, but simple solution.
• The neuroscience behind mindfulness is clear. Breathe, smile, stand tall. All work.
• Python itertools.combinations/permutations.
• Google coding interview #2 today. Went well.
• Lunch with briley at wing ferno.
• Put new (2021) registration on the BMW.
• When doing DFS or BFS, you can keep track of the depth/level. Just store each node as (val, depth) instead of (val), then increment it within the (for neighbor in neighbors: queue.append()).

• Citadel onsite confirmed, next Thursday. Choice of Chicago or New York. Jack said both offices were great. Chicago is a shorter flight, so we'll go there. Fulltime can change after, if need be. Fly out midday wed.
• Very quick (2 question) interviewer feedback form for Amazon.
• California would be the 5th largest country economy, after Us China Japan Germany.
• Lots more system design videos. I really do love this stuff.
• Alphabet hit 1 trillion market cap today.
• Probabilistic solutions for hash collisions.
• When counting or checking existence, the natural solution is a hash table.
• The solutions here: use multiple hash functions, then compare. This makes you robust to collisions.
• Count min sketch algorithm.
• Used for counting how many times an item appears in an array.
• Iterate over the array, use multiple different hash functions on each item, then calc the min of all the integer counts produced by them.
• Bloom filter.
• Used to check if a record exists already (think "username taken"...).
• As each record is created, run it through multiple hash functions and put entries in a bit array.
• As a new one comes it, run it through all hash functions. If there's already an entry for all, then you're 90% sure it's already taken.
• If one output doesn't exist, then you're 100% sure it is not already used, because you would have put an entry for it when the record was created.
• These typically use efficient hash functions and memory-efficient data structures.
• Allows you to do something like pass a bloom filter clientside so that a user can get immediate feedback in the browser if a name is taken. Just have to pass the K hash functions and a bit array of the possible outputs.
• Rate limiter design.
• Just think like github's "remaining-requests" to the api. Keep a db which has a mapping of user: tokens. When they're all used, reject requests.
• You'll usually need to store timestamps too, so you know how the usage has been spread over your particular window (last minute/hour/whatever). Just like a queue: [ts1, ts2, ts3...]. You can count them as desired. Sliding window.
• Disney finally got back to me, they'll schedule a phone interview for ASAP. Reminder the comp package is much less than the others: ~190k base, no equity, contract 18mo, half benefits, some disney perks like movie screenings. I'll see how the phone goes, then balance with my current burden of the others before committing to an onsite interview. They're glendale, which is a bit more convenient than the remotes.
• Nosql databases can be indexed for speed as well. It's usually a compound index of multiple keys mushed together. Remember that an index is just something you can query quickly because it's sorted.
• Sam Harris is so good: https://www.youtube.com/watch?v=LRm_H158qc0. Be grateful for things that haven't happened. Easy way to stay in the moment, impervious to negative emotion.
• Downloaded Waking Up. Will read after jim simons' bio.
• To create the floodfill feature (the paint bucket in mspaint), just use a graph traversal (either BFS or DFS) where the neighbors are all 4 nodes in the cardinal directions. If the value is the same, update to new color.
• Practice problems.
• Focused mostly on DS&A today. Tried a new website for diversity, geeksforgeeks.
• Good summary of everything: https://www.geeksforgeeks.org/must-do-coding-questions-for-companies-like-amazon-microsoft-adobe/.
• In general, geeksforgeeks has pretty good tutorials/descriptions. The text is better, the IDE/leaderboard/etc is worse.
• Poison bottles and rats to test. Each iteration, have the rat drink half the bottles. Reduces it to binary search basically, logn.
• Binary trees. Find max width, height, sum of columns, etc. Just do -1 for each left traversal, +1 for each right traversal. Keep in a global hash table or something. Then you can run whatever stats you want on it.
• Ended up switching back to leetcode. Geeksforgeeks has a bad interface. It requires you to supply your own input/output reader/write, rather than making it class or function based with prefilled templates.
• https://leetcode.com/problems/continuous-subarray-sum/. This one wasn't easy. Find modulo of cumsum, check to see if two are the same, which means the cumsum between them is divisible by the target.
• https://leetcode.com/problems/baseball-game. Easy.
• https://leetcode.com/problems/top-k-frequent-words. Easy. Remember you can heap for nlogk instead of nlogn.
• https://leetcode.com/problems/spiral-matrix. Walk 2d grid in spiral. Just define the 4 directions, loop through them. Take whole row, take first or last item in all rows, and reverse as necessary.
• https://leetcode.com/problems/median-of-two-sorted-arrays/. Didn't do it, just reread. This is a great article: https://medium.com/@hazemu/finding-the-median-of-2-sorted-arrays-in-logarithmic-time-1d3f2ecbeb46. The problem is way out of scope for an interview.
• https://leetcode.com/problems/longest-palindromic-substring. Did the "expand around all centers" way.
• https://leetcode.com/problems/trapping-rain-water/. Trapped rain water, iterate once from left and once from right. Calculate max and deltas, then compare and take min. That's the trapped water.
• https://leetcode.com/problems/rotate-image/. Rotate image. If you're allowed to use extra space, just fill in the new matrix. Write a couple down to see the pattern. New value is just (j, n-i-1). To do the rotation in place, go onion layer by onion layer and do 1, 2, 3, 4 = 1, 2, 3, 4.
• https://leetcode.com/problems/jump-game. Did DFS. There's a much cleaner solution, basically start at the end and check if you can reach this position, then iteration back through the whole range.
• https://leetcode.com/problems/merge-intervals. Good example of making assumptions and testing them out. Did it vocally. Would have clarified with interviewer if live, rather than breaking/fixing with testcases.
• Applying locks, mutexes, semaphores to a concurrent program in order to avoid race conditions: synchronization.
• Priority inversion: when a high priority tasks is waiting for a low priority task to finish. Priority inheritance: when the low priority tasks becomes high priority bc the next job in the queue is high priority, to make sure nothing takes it away.
• In python2, xrange was more memory efficient than range. It wouldn't create the full list, it would lazily generate them as you need. In python3, range is xrange.
• Python's heapq.heapify will take a list and make a heap in linear time (in place too!). There's also nlargest, nsmallest.
• pop(0) for first element, pop() for last.
• collections.Counter(arr).mostcommon(k) is very useful. It doesn't have a nested sort, after count it's arbitrary. To enforce alphabetical or something on the second level, just write it yourself.

• Letsencrypt sent out deprecation notices for ACMEv1. Starting June 1, only ACMEv2 standards will be accepted as sufficient certs. I'll have to upgrade the jrcs/letsencrypt-nginx-proxy-companion (running certbot) before then.
• Smoked the 15lb dry-brined prime brisket. Garlic, mustard, paprika, pepper. Post oak and cherry. Honey.
• Ran out of 18" aluminum foil. For a big cut like brisket, it definitely leaks with 12". Ordered more.
• Plaid connects apps like venmo, robinhood  to accounts like citi, boa. Visa just bought plaid for 5.3b.
• I get python weekly and founder weekly, but I'm subscribed to nosql and never get that newsletter?
• Airtable makes a kinda cool product. It's a database/spreadsheet combined. Standard backend but offers a full frontend gui like google sheets. This exists in many stacks, but it's usually 2 products.
• Kadane's algorithm for max some of contiguous subarray.
• Basically at index i, consider all the subarrays up to index i, starting from every index before it. Get the max sum of those. Go to the next index. It's just the previous answer plus the number at the new index, or the previous answer itself.
• local_maximum[i]= max(local_maximum[i-1], array[i] + local_maximum[i-1])
• Simple recursion problem.
• Rewrote some graph traversal algorithms. Raw BFS/DFS easy. For pre-order and post-order traversal (types of DFS), use recursion. Just add the node to `visited` before or after the recursive call. For level-order (BFS), use iteration.
• For python remember the distinction between the subprocess and multiprocessing modules. Subprocess is for calling other programs, running something else from the shell, etc. Multiproc is for adding concurrency to your python program.
• Boston is ~200 miles northeast of Manhattan.
• If you're alive, you can breathe. If you can break, you can speak :)
• Threading is better for IO-bound tasks. Multiprocessing better for CPU-bound tasks.
• Read quite a bit about py3's async capability. https://realpython.com/async-io-python/.
• Slightly different than the parallelism/concurrency of threading/multiprocessing. Its async features (like asyncio.sleep()) are self-aware of when they're idling, efficiently giving the event loop to another coroutine during that time.
• Think of a standard web service, able to handle multiple requests in parallel. Asyncio is that capability, but for your program. It's very different than multiprocessing, which has a given workload to do and wants to split it among many workers to make it complete faster.
• async def foo(): tells python to create a coroutine. Within that function, say await bar(), instruct the event loop to go work on something else until bar() returns.
• Websockets.
• These build on that exact same async infrastructure. Before py3's asyncio, there was eventlet and gevent. Very similar principles.
• You basically have a route that's decorated slightly different than a REST route, and it keeps a websocket connected to the client. Every time a new message appears, it will perform the logic in that route. Example: flask-socketio. Just @socketio instead of @app.route.
• You can also go low-level yourself and define an explicit `async def my_coroutine` with `await websocket.recv()`. The websockets module comes with a very basic server that can persist the connection. https://websockets.readthedocs.io/en/stable/intro.html.
• No matter what, this manifests itself eventually as a data structure in python. Think of it like a list, in memory, that an async service is continually appending to under the hood. Do whatever you want with it, it's just a regular old list to your python app.
• Capsaicin is spread by birds. You might think seeds from chili plants don't get spread around by animal scat because they don't want to eat them, and you'd be right; with the exception of birds, who don't have teeth and just swallow the seeds.
• Watched Shimmer Lake. Dwight!
• Worked again with javier, jack, and tara for the next rounds.
• Scheduled google for friday.
• More general prep. Checked linked profiles of Monday's Amazon interviews. Watched about 10 system design videos, each ~30min.
• Lots from this channel for system design: https://www.youtube.com/channel/UCn1XnDWhsLS5URXTi5wtFTA.
• Locks.
• Even if the transaction isn’t a single db statement (like a deposit) but has multiple steps (like a transfer), you can still lock. A transfer is a withdrawal + deposit + commission, for example. If ANY of those 3 steps fails, have it undo the whole chain.
• You can queue transactions such that if A has a lock while B tries, B will try again in a few seconds when the lock is freed. This is called retry logic. You can write it with TRY statements in SQL!
• The full queue of transactions to write is called a write-ahead log. Like a task manager built into the database.
• Most locks will only lock the scope that's being changed (only that row). Some locks will disable write but allow read during the change. Some locks will disable both.
• Pessimistic lock = the standard one you’re thinking of. Lock a row, change some data, release the lock. The safest way. Better for joint sets of data, many conflicts.
• Optimistic lock = check the timestamp / version / hash at the start of the transaction, and then again when you’re ready to write back. Only write back if they’re the some. It’s less restrictive, and allows multiple people to read, but it basically favors fasters changes. Slow changes get clogged. Better for disjoint sets of data, few conflicts.
• Locks can be distributed, just like the databases themselves. You can have a cluster of redis nodes (direct duplicates for backup), each storing the mutexes for writes to the primary cluster. microservicenodes -> lockmanager -> caches, just like client -> loadbalancer -> microservicenodes.
• Locks have timeouts to ensure some dead node doesn’t halt the system.
• Put timeouts on everything! Responses, db locks, everything.
• A single char usually takes up 1 byte, so a string of len 4 is the same as an int for a 32bit system. A full sentence, maybe 100bytes. A paragraph, maybe ~1KB. A large doc, maybe 1MB. Compression helps too.
• CAP = consistency availability and partition tolerance.
• P is how the system reacts when nodes can’t talk to each other. Examples: two shards drop a connection, master fails the replication to the slave....
• The whole point is that in the presence of partitions (distributed systems), you can only choose one of (consistency, availability).
• Master-slave is AP. You might have inconsistency, where a write replication might take longer than the next read.
• Many equal nodes is CP. They might handle different tables (vertical partitioning), but they back each other up. This is a distributed datastore. Banking. REST. Locks on everything.
• Cassandra has something called "replication factor" which is how many other nodes to split the backup of a node on.
• Distributed transactions.


• Have the microservice itself (or a separate orchestrator/coordinator) keep a record of all planned queries/writes in the chain of the request. It needs to regard all of them as a single transaction. If any of the individual reads/writes to various other dbs fails, it needs to rollback the entire thing. You might hear this called "3-phase-commit" (can commit, precommit, do commit).
• Consistent hash ring.
• Hash tables under the hood. You might think the keys are just assigned to registers, 0-2^32 or something. Then you hash the input and simply lookup by key. This would work, but if anything changed (like the # of total memory locations), you'd have to remap your table to the new modulo.
• Instead, just make the keys random numbers. Then during lookup, pick the first key that's greater than your hash output. If you reach the end, just take the first element. Then, since you're finding the closest neighbor instead of the exact key, it's robust to remapping.



• While 50k requests/sec to a website is very large, to be considered “very large” in the world of queries/sec you must go even higher. Large apps are over 1 million queries/sec.



• Caches.
• A global cache can be as big as terabytes for large websites.
• Cache latency should be <10ms.
• LRU is a very common cache eviction policy.
• Under the hood, a cache is just a message queue with an event loop that has a threadpool at its disposal (and access to ram).
• Fault tolerance in a cache is usually just regular snapshots to disk.
• Since caches are typically event-driven, you can also take the logs and just replay all the actions.

• Watched I Think You Should Leave with Tim Robinson. Some good sketches, some average.
• LSU national champs.
• Bought, trimmed, brined a $15 prime brisket.
• Since I have the tax/house loot sitting in my BOA, I'm temporarily eligible for preferred rewards. Not gonna get another card or anything, but there are some small perks like no ATM fees at non-BOA ATMs, etc.
• Paid the $53 parking ticket for the garage next to tower 12 last friday.
• HDFS = hadoop distributed file system. Part of the Apache Hadoop suite.
• Did a cloud product comparison review between the gigantic stacks of AWS vs Apache vs Azure.
• While I was working on the Netflix takehome assignment, the position was closed (over the weekend). Dunno if that meant filled or headcount-cancelled. The hiring manager emailed personally and said that she liked me and wanted to personally forward to another hiring manager. I looked through all the open engineering reqs in Los Gatos / Los Angeles and asked for the python runtime group.
• I really do like 23andMe.
• Forget privacy, murder charges, general data paranoia. Accountability is good. More data is good. And this data is about as fundamental as it gets.
• Leads to connections. Leads to drugs. Leads to wellbeing.
• It's a fantastic business model. You're paying them to give them data, which is their actual product.
• Average US credit score in 2019 was >700. Higher than I thought it would be.
• Google and FB building little towns of housing complexes near their headquarters to offer affordable housing (+ convenience, - work/life balance). https://medium.com/cxo-magazine/google-and-facebook-are-building-the-ultimate-perk-housing-3ec8ba3c4f6b.
• Current vegas money lines, niners and chiefs both favorites at -320.
• The most career interceptions is 81. There used to be a lot more. The most for active players is 35, Richard Sherman.
• Features like search autocompletion should have a response time <100ms.
• A trie (pronounced try) is a specific type of tree, where the nodes are characters.
• For the english alphabet with 26 letters, it's just a tree with:
• Root node = null.
• Each node can have up to 26 child nodes, one for each next letter.
• Traversing down any path spells a word, (the path being the return string).
• With these properties, you can look up a word quickly. O(l+n), where l is the length of the prefix that you're checking against, and n is the number of nodes underneath that trie subset (because you have to traverse all of them). Then you'd usually do a sort, for klogk more, where k is the length of the valid words you found.
• This is used for stuff like autocompletion / look-ahead text.
• Jeddah Tower will open this year and surpass Dubai's Burj Khalifa as the tallest building, but then the Dubai Creek Tower should open the year after that and take the crown back. It costs over $1b. It will be over 1km tall.
• Remember to consider a timescale. If a particular computation is expensive, but the user doesn’t need accuracy of the data down to a resolution of seconds/minutes, just compute it and cache it once an hour or something!
• Jeop GOAT day 4. Ken won!
• FTP = file transfer protocol.
• I didn't know LinkedIn was the original creator of Kafka; Apache is simply the steward nowadays.
• Wanted to subscribe to highscalability's RSS feed, but gmail doesn't offer native support for RSS (anymore). This is crazy. I don't want to have to create an account with a standard newsreader and use it as a middleman. I'll just check the blog manually, but it's a shame to have to do so in 2020.
• Bought a new food processor.
• Amazon called, basically said that hiring was a consensus, but they were considering bumping up from II to senior. They wanted to get more data points to confirm, so they requested a second, shorter, onsite interview with more senior developers.
• 2hrs instead of 6.
• Same format, half behavior and half technical for each hour. 1 would be another problemsolving question and 1 would be another system design question.
• The behavioral halves would be split among a specific 4 of the LPs: think big, invent and simplify, be right, hire the best.
• Overall, I feel good about this? It's more work, but it's the chance for an early promotion. 2hrs for a stressful onsite is easier than working for a promotion internally for a year. In the worst case, I hope there's no offer retraction for the lower level if the second onsite goes poorly, for some strange reason. In general, I'm happy to see that the interviewers recognized some good principles and suggested higher leveling without my solicitation - the exact opposite of SpaceX :). Moving forward with gratitude, no matter what happens.
• Emailed Citadel and Google to let them know so that we can accelerate those tracks to stay in sync.

• 6 more leetcode problems.
• Remember that sets are mutable in python, and therefore are not hashable.
• If you need to solve a problem with nested lists/sets, and you need to check if an inner list is already in the outer list, it's slow. It's much faster in a hash table (set or dict).
• To solve, make the outer and set and the inner a tuple. You can leave the inner a list for whenever you're modifying it, but then you can convert to tuple when checking/writing to the outer set.
• Amazon onsites.  6 1-hr interviews. Notes in drive. Enjoyed it.
• Jack mentioned that Citadel is still putting together the onsite panel, so it'll likely be next week not this week.
• ZYME up 4% and TSLA up 11% today (to $530, w t f). ZYME had a -15% dip right after open, lasted for about 30min. Not sure what caused it, some afterhours behavior, who knows.
• Collisions. Two writes at the same time. Databases, both relational and in-mem, have locking mechanisms. Under the hood, there is a mutex that will keep transactions from overwriting each other (if it’s an ACID database). If you have two REST calls trying to book an airbnb, the db will confirm one as first and return the write transaction; the other will fail. The service should then tell that to the user.
• Remember if your DB doesn't have native support for a date/datetime object (like redis), just convert to an integer via seconds-since-epoch.

• The MD5 hashing algorithm resolves to 128bit values.
• CRUD create read update delete.
• Final prep for amazon.
• Few questions on glassdoor. Mostly behavioral, some algorithmic.
• Few questions on careercup. Technical.
• All the big hitters in my gdrive:
• technical: a&ds, aws, databases, debugging, general software, linux, speed this up, sicp, system design, web.
• behavioral: my questions, their questions, amazon leadership principles, logistics.
• Few leetcode problems:
• https://leetcode.com/problems/rectangle-overlap.
• https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array
• https://leetcode.com/problems/search-in-rotated-sorted-array.
• https://leetcode.com/problems/jump-game-ii.
• https://leetcode.com/problems/combination-sum/.
• Signed the NDA.
• Anything >= 1000 requests / second is usually a heavy load. For a smaller system, a few hundred can even be heavy. Google processes ~50k searches per second.
• Can we get a favicon on bigocheatsheet pleeeease. https://www.bigocheatsheet.com/.
• Last 2 divisional games, so conference championships will be Titans vs Chiefs and Niners vs Packers.
• For recursive problems, you can:
• Naive: Generate every single permutation/path and then run through and check each as valid/invalid.
• Backtracking: Try a value, verify, drop if wrong and move on to the next. Continue if valid.
• Powers of 2:
• 2^10 ~thousand
• 2^20 ~million
• 2^30 ~billion
• 2^40 ~trillion
• Read sequentially from:
• Disk at ~30 MB/s.
• SSD at ~1 GB/s.
• Memory at ~4 GB/s.
• Fantastic overview of system design: https://github.com/donnemartin/system-design-primer.

• Traversing paginated APIs is usually easiest with `while resp.links.get('next'):`
• Finished the netflix take-home coding assignment and submitted. It was a bit larger time ask than I'd prefer for this stage in the interview process.
• There's a few good examples of raw stuff in here. Pagination traversal, greedy page counts, custom manual mocking.
• 2 divisional round games! Niners. Went to the pier to watch with the sbsc season finale.
• Tox can have issues finding output data from pytest-cov. Just add usedevelop=True or setenv=pythonpath={toxinidir}.
• type('', (), {})() for an empty object you can set attributes for, etc. I know, there should a single call like object(), but you can't write to that instantiation.
• Remember, classmethod is run on the class not the instance (class()). staticmethod doesn't get self passed, it can be run without an object.
• Always patch in the namespace of module you're working. This is true even if the function lives somewhere else! @patch('mypackage.mymoduleimtesting.myfunc').
• Hahaaaaaaa: https://www.youtube.com/watch?v=4Pr_E8Tz7RE. Amazing game show idea. https://en.wikipedia.org/wiki/Killer_Karaoke.
• Went through notes/career/software and split up some of the bigger docs: general software, web. They had smaller partitions that made more sense (dbs, etc). Consolidated all the books too: SICP, PIE, CTCI, DS&A.
• 30 incredible summary documents in total now.
• Grouped all behavioral questions into categories and then meshed them with an independent bank of my experiences/skills/projects/interests. Helped organize the responses, which should make them easier to bring to memory for the amazon leadership principles.
• Wrote a system design diagram for all the nodes in autotest, from memory. Was a good exercise to connect new design knowledge with old (semifamiliar) architectures.
• Watched another couple design videos on uber and tinyurl.

• Petty released the sbsc winnings, $700 for 1st place, $50 for 10th. End of season party this saturday.
• Path-planning is an np-complete problem. The traveling salesman is an np-complete problem. Dijkstra's algorithm alone is solvable in polynomial time.
• Traveling salesman is just visit every city in the country once in the shortest path. In other terms: visit every node in a graph once, where the edges can have different weights (like distances between cities).
• ! to run shell commands from ipython.
• Spoke with the google recruiter. Gave her my feedback from the other day, and combined with the interviewer's fb, decided we'll do a 2nd phone interview. This is good - I wanted another datapoint. We'll do it next tuesday.
• With py requests, can pass it in directly with ?key=value or via params={key: value}
• You should typically use the rel links for next and prev when dealing with a paginated API, rather than just looping over an integer range and building the URL yourself. Sometimes the url will change.
• Dentist and haircut.
• Interesting report on the (primarily) medical state of psychedelics, and the effect on market investments we'll see in years to come: https://www.reddit.com/r/investing/comments/emkhwt/2020_psychedelic_industry_insights_report/.
• Went through a bit of highscalability. It really is a great blog.
• Jitter can be a good thing. Sometimes it’s best to intentionally add a bit of entropy in. Helps avoid thundering herds.
• Spent most of today on the Netflix take-home assignment. Private github repo.
• breakpoint() is only available in py3.7 and beyond, it's still import pdb; pdb.set_trace() for all vers before then.

• Lots of system design practice today.
• NoSQL vs SQL.
• NoSQL is useful when you always need all information about that row. SQL better for select x, NoSQL better for select *.
• Don't need NULLs. Just skip keys in the nosql json. In this way, the schema is basically flexible per entry.
• Good for analytics.
• NoSQL is more expensive for updates, and doesn't guarantee ACID.
• NoSQL is more expensive for reads. While you can grab a whole blob easily, getting only one col across the whole db is slow.
• JSON nesting is nowhere near as structured as the the relationships (FK, many-many, etc).
• Joins are weird. Not easy.
• Overall: if your data is always read/written in blocks, not individual cols, nosql is good.
• Key-value is just key-value. Document nosql can have structure and nesting within (JSON usually).
• Examples:
• Document nosql dbs: mongo, couch.
• Key-value nosql dbs: dynamo, redis.
• Wide col nosql dbs: cassandra.
• Graph nosql dbs: neo4j.
• 10m call with Jack to set up the onsite with citadel, a few notes:
• Will's team is interested, but there are a few others as well (equities...). Interview might be at one site, but fulltime might be at the other.
• You don't need to have expertise in finance, but you need to show interest. The projects of last year are great examples.
• ACID is a db transaction property for transactions. Atomicity, Consistency, Isolation, Durability. Financial institutions absolutely require this. Something like a metrics server wouldn't.
• Availability is another important metric, but isn't part of ACID.
• Can shard by ID, or by location, or whatever is the most convenient/divisive/common entry point.
• JOINs get expensive if they're cross-shard.
• Master-slave architecture. Master gets all the writes, slaves update to match master every X sec/min/hours/days, slaves get all the reads.
• If master goes down, one of the remaining slaves becomes master.
• Still dumb that its formal name is master/slave in 2020.
• Remember generators just return before they're finished executing. Could be 3 lines straight with yield 1, yield 2, yield 3, but it's usually a loop within the function. Just returns partial progress. Then you can iterate over the returns as they come, or call next(my_gen_func) to walk through manually.
• Confirmed and planned the logistics for the amazon onsite on monday.
• Need to print the NDA before then.
• Consistency is very important. Do you want mutexes on everything read/write such that the data returned is always exact? This is much slower and much bigger. How much data can afford to be out-of-date? How much time must pass before data is considered obsolete?
• If I scale a microservice horizontally, the machines need to talk to each other. RPC. If I scale vertically, it's all local still. IPC. RPC is slower, and more complicated to manage. But it's usually cheaper, and it's boundless, and it's more resilient to failure.
• Remember: event-driven/pubsub/messagequeue/bus designs are worse with consistency, but better with availability. You’re distributing events and aggregating results. If you need higher fidelity responses, go with more of a request/response (timeout) design. Slower, but more integrity in consistent data.
• Event-driven is easier to test, play, rollback, because it’s structured and sequenced. Kinda like react/redux.
• In-mem solutions use snapshots to get persistence, but they're obviously at a certain frequency and so you open risk for data lass.
• Postgres has nosql features as well, it's just used less commonly. You can store docs just like mongo.
• Amazon 1hr onsite coaching call, general to all onsites.
• Basic fundamentals. The one thing I usually have to remember is: Ask questions to clarify the problem. This especially applies to design questions, but can also apply to coding questions.
• Amazon 30m final call, specific to me.
• Schedule: (each 1hr)
• 2 technical: Problem solving, DS&A.
• 1 nontechnical with hiring manager. Customers, communication, dealing with adversity.
• 1 lunch. Culture. Ask questions. Relax.
• 1 technical: Logical and maintainable code.
• 1 technical: System design. Scalability. Operational performance.
• Each will have 2 leadership principle questions (behavioral) as well.
• General:
• Ask clarifying questions. Think out loud.
• Start with an easy solution. Brute force, whatever. Then expand. Take bite-sized bites.
• The interviewer is your coworker! If you start to panic, look at them like a collaborator.
• Always consider priority. A bank withdrawal is much more time-sensitive than an email notification. This could affect the ordering in the message queue, the rate limiter thresholds on various notes, the load balancing, etc.
• When realtime accurate data is cumbersome, you can trend or interpolate data, based on history, to give an estimate. This is true for something like the viewcount on popular youtube channels. Would take too much to show exactly, but is easy to show ballpark.
• Thrashing - when a cache is inserting/deleting too quickly.
• Going through this process of interview prep has been interesting. I feel like I've consumed what must equate to at least a handful of semesters in fundamental CS courses. This accessibility of information makes me wonder about the future of education. It's been maybe 6 weeks and $0, whereas the university equivalent would have been a year and $30k. Will our grandchildren get in-person degrees? I'm not so sure this model is viable in the future.
• Ordered more dry-erase markers for whiteboard practice at home. Arrive tomorrow.
• Netflix splits every piece of content into small atoms, stored with different permutations of codecs and resolutions. That's why viewing is seamless when your internet quality changes, or you select a new one; it will simply then fetch the right chunk instead of the whole movie again. The amount of chunks it preloads into the future is based on heuristics, by how much people typically jump during this movie. If a lot -> preload only a little. If a little -> preload a lot.
• Did a practice system design from scratch for instagram/twitter.
• It looks like neither grubhub nor yelp do group carts anymore? I remember loving this feature. You'd just send the link to anyone, they'd look through then menu and add, then be responsible for the finances individually when the order was sent.
• You could store images in a db as blobs (binary large objects), but it's almost always still best to put them on a file system elsewhere (CDN, s3, etc) and keep only a URL to the file in your db.
• A client-server relationship is always initiated by the client. Request -> response. The protocol for this is http.
• A peer-peer relationship can be initiated by either side. You can use a general socket (TCP) or a protocol specific to your use case. For chat (whatsapp, fb messenger, etc) the protocol is XMPP - extensible messaging and presence protocol.
• It’s ok to have gut instincts. It's even ok if they're wrong. State them, and then state that you’re going to analyze them now. Do the verification out loud. A raw, transparent thought process is worth a lot.
• Designing google search.
• First, just think about keeping a big dictionary. As people add sites to the internet, it adds words and phrases to the dictionary. You have a rough mapping of what is out there.
• Then think of it like a gigantic hash table, or an indexed db. Look for the word (or words) in the search, and return the direct results.
• That might not be all though - look for misspellings, common associations, and other relatives. This is where association algorithms come in. They're usually based on prior results. There are many other keys to consider: location, how recent, etc. There are also blacklists for known spam, risks, more.
• Then, you have a list of possible options. How do you rank them? What do you show at the top of the list? This is another ranking algorithm. Again, usually based on prior results.
• How do you manage the size of the dictionary? MapReduce. Basically shard the hash table by characters (since those are they keys that the user is typing), farm out to the all the appropriate nodes to their more-efficient subsearches, then combine the data back.
• ~1 in every 7 strings typed into the google search engine has never been typed before!
• Write-ahead logging is when the action is logged before it is performed. Think of it in the context of a database write operation - this handles the atomicity and durability principles (of ACID).
• Third day of jeop goat.
• Replication types. Consider a db mirror, or a master-slave.
• Snapshot replications. Vompletely copy the full data over, like a backup or a pgdump.
• Transactional replication. Replay the transaction on each slave.
• There are more, but don't worry a ton about them.

• Think about if a system needs to be read-heavy or write-heavy. Something like twitter is much heavier on read operations.
• An index is placed on a specific column in a table so that you can perform lookups on that column more quickly. It's not the primary key, although you can think of the PK as an index if you know the ID, because that's the search val. More generally, it allows an indexed column to be sorted efficiently on the backend, so that lookup is binary search (logn) instead of n. The downside: this extra data takes space.
• Think of it like a dictionary (an actual physical book with word). Indexing it would mean adding pages for each letter in the appropriate place, with little tabs that jut out. You can find your section much faster, but they take a little space.
• PUT will update the data in that spot or create it if it isn't there. It's idempotent. POST just passes data to the service. The service can do whatever it wants with it. It's not idempotent. It's more generic.
• Citadel interview got a callback for onsites. Call tomorrow for scheduling.
• YAML is a superset of JSON. Can do more, but is bigger. Looks better, too.
• Testing pyramid: unit -> integrated -> e2e.
• Netflix phone interview.
• Great convo. Took a one-pager of notes. Just phone, no coding. 1hr.
• Discussed the INTERNET SHIELD, market viability in the future of streaming, attrition, culture.
• Her team directly aligns with my automation tools / devops / autotest / sx-setuptools experience.
• Next steps: take-home assignment, build an app. I said I'd submit by Saturday.
• DHCP for dynamic IP allocation.
• Perf is a main linux profiler.
• "Computers perform tasks. Machines should be solving problems."
• Practice problems:
• https://leetcode.com/problems/search-insert-position. Easy.
• Remember runbooks for ops/support. All stacks/apps should have a list of common tasks or inquiries, each with an explicit set of steps to respond with.
• Slight tweaks to my resume. I still like my abridged resume more, although nobody else does. It's just so concise: 20 bullet points to summarize my life so far.
• TSLA is absolutely skyrocketing. Hit ~500 today. Its market cap is now 85b, which the highest for any american car company in history. I bought 2 shares on margin just to have fun with it. The emotional roller coaster is worth it, regardless of final profit/loss (since I jumped in ON a surge woohoooooo).
• Remember margin trading on RH is interest-free up to 1k, then 5% after that.
• Watched a lot of this guy's channel on system design: https://www.youtube.com/channel/UCRPMAqdtSgd0Ipeef7iFsKw. I like em.
• Athletes say a lot of dumb stuff on social media. It seems easiest to critique them for poor decisionmaking/tact, but in reality it's probably closer to: giving a huge stage to people on a career path where stage presence is not a required skill.
• Jeop GOAT day twoooo.
• Google phone interview.
• Was 45m. The guy was cold, didn't really reply to anything or change tone. Accent was tough over the phone.
• Didn't introduce himself or the team, jumped straight into a coding question on the google doc.
• We did a 2 subsequence comparison question. I solved it, but still had a bad taste in my mouth.
• We didn't discuss previous experience, scaling services, managing projects, writing apps.
• Big turnoff for Google, when my interactions with all other folks from Amazon/Netflix/BMW/Citadel/GitLab/Disney have been very positive.

• There's an online shazaam service called acrcloud. https://www.acrcloud.com/identify-songs-music-recognition-online#record-div.
• That old ACB call order which expired would not have broken even if it were filled. Coo.
• Dis is still about the same plateau after the d+ surge, when I sold. Coo.
• Was digging this article until they starting talking about "electromagnetic radiation" affecting your sleep, suggesting that you ground yourself before bed. Then on to light therapy and all sorts of good stuff: https://medium.com/swlh/sleep-like-a-pro-whats-the-deal-with-deep-sleep-and-how-to-get-more-46dad5da233d.
• Interesting discussion in a drug from KRTX against alzheimers: https://www.reddit.com/r/investing/comments/elclxc/im_a_physician_and_im_long_krtx_dd_inside/. This sector is crazy, I like it. Being in the industry seems to provide a ton of inside info, more than relative to being inside others even.
• Ricky Gervais' opening monologue for the golden globes was great.
• Practice problems:
• https://leetcode.com/problems/longest-valid-parentheses/. Back to try a few leetcode. Much shorter problem statements, love it.
• Convertible arbitrage: taking a long position in a convertible security and a short position in the underlying stock. It's a hedge tactic.
• Coding interview with citadel. 1hr. Phone and coderpad. Prepped. Took a lot of notes during. 2 behavioral questions, 2 coding questions. Overall impression: good in both directions.
• Said "see ya later" to hang up without even thinking twice about it, like I always do. Had some regret later; might have been interpreted as overconfidence or something. Need to pay attention to details, even ones this small.
• Created a doc with all the resources I've used over the past month for interviews.
• Generalized definition of a greedy algorithm/problem: start small and build the problem up, taking the (new) local optimum each time.
• Went back over everything in gdrive/Notes/Career/*. Some fantastic refresher content in there. Mostly technical software, some workplace environment, some interview logistics. Being diligent about documentation my whole life has paid off.
• Quicksort/mergesort/heapsort nlogn. Heapsort constant mem, others linear. Bubble slower time, const mem. Radix/bucket can be faster than the main 3 if n is small.
• Anything hashable can be used as keys. All the immutable python built-ins qualify, including tuples (whereas lists don't).
• QR = quantitative researcher.
• CDS = credit default swap. Kinda like an insurance policy on credit card debt, can get it backed by another investor for a price.
• Supercontest.
• Changed the home page (the root route /) to the leaderboard instead of your weekly picks.
• Scheduled haircut for friday, before onsites.
• First day of jeopardy GOAT tournament.
• Explored http://highscalability.com/.
• Looked up a BUNCH of resources on system design. I feel pretty good about behavioral and algorithm/datastructure questions now - system design is the last piece. Took a bunch of notes, did a few examples. Will focus on more in-depth ones over the next few days for on-sites, but I feel ready enough for the brevity of phones.
• Went over 27 common "tell us about a time..." questions and wrote responses to each. I then tried a ~60s spontaneous response to each. There are definite overlaps. A good core of 10 responses can apply/stretch to all 30.
• I did generics, and then I did concretes for Amazon's leadership principles. They're obviously the same set/class of responses, just need to have them in the toolbox ready to connect.
• Pubsub research. Not great in consistency. Latency means timing-related transactions are not great. Subscribers might receive messages at different times. Better for event-driven architectures like games.
• JSON serialization/deserialization gets less efficient as the size gets larger and larger. This is where you might pick another data transport for your API, like protobuf. You can have an endpoint return a protobuf object instead of a json object. Very similar.
• Typically, use json when talking to a browser. Use protobuf when talking to a service.

• Practice problems.
1. https://www.hackerrank.com/challenges/max-array-sum/problem. DP. Iterate through the array and notice patterns. What info do you need to hold on to? What are the possible answers? Keep track of the max by index and just walk through.
2. https://www.hackerrank.com/challenges/abbr/problem. Easy to get most the cases, but hard to get all. Remember for DP you can sometimes build a 2D matrix of 1s and 0s and traverse it (like longest common substring/subsequence).
3. https://www.hackerrank.com/challenges/candies/problem. An interesting one. Iterate through an array, once forward, once backward, and count the rising edges. Match them by index and take the max.
4. https://www.hackerrank.com/challenges/decibinary-numbers/problem. Didn't do it. Wrapped my head around it, but didn't implement.
5. https://www.hackerrank.com/challenges/min-max-riddle/problem. A cool problem. What mapping do you need to solve this? Can find the INVERSE of that mapping?. Swapping keys/values in a dict is easy.
6. https://www.hackerrank.com/challenges/castle-on-the-grid/problem. Used a graph. Build an adjacency matrix of the 4 cardinal directions from each node and remove the ones that are off the grid or on an X. Then it's simply a shortest-path problem. Use BFS (queue).
7. https://www.hackerrank.com/challenges/poisonous-plants/problem. Got the easy solution.
8. https://www.hackerrank.com/challenges/ctci-bfs-shortest-reach/problem. When using BFS for shortest path, remember to slice the list for each neighbor, so you're not overwriting the root path. And just logically think through everything. Created visited set() and queue deque(). while queue. Get path. Get node. If node == end, exit. If node in visited, continue. Else, lookup the next nodes in the adjacency matrix, add to path, then append to queue.
9. https://www.hackerrank.com/challenges/find-the-nearest-clone/problem. Getting good at the graph problems. It was a less-experienced topic for me before this.
10. https://www.hackerrank.com/challenges/ctci-connected-cell-in-a-grid/problem. Could do DFS or recursion or iteration.
11. https://www.hackerrank.com/challenges/matrix/problem. Fun problem. There's actually a bug in a unittest in this one.
• Done with the hackerrank kit. I've probably done 100 practice problems not in total across the platforms.
• Confirmed the phone interviews this week.
• Dijkstra's algorithm is used to find the shortest path between two nodes in a graph with weighted edges. If you assigned no weights to edges (such that they're all the same, 1), it reduces to plain old BFS!
• BFS is exponential (based on how many neighbors each node has) in time and memory. It's great for small problems, but solving something with a state space like a rubik's cube is huge.
• Sets and lists are unhashable in python. For most problems, you can just convert the (i, j) coordinates to a string or something similar.
• collections.deque is faster. This whole package is great for performance. defaultdict is awesome. Counter is also really useful for counting stuff (like frequency of char in str, or others).
• When using BFS for shortest path, put PATHS (lists of nodes) instead of nodes into the queue.
• Append the new nodes to the right of a queue or a stack. But in a queue, pop the node off the left to search next. In a stack, pop the node off the right.
• Run, pull, sauna, meditate.
• Checked the snail mail. Probably 100 items. Everything was complete garbage except for 1 dmv registration. The signal to noise ratio is laughable (and wasteful of paper / people resources).
• Renewed the BMW registration. $150 total. The DMV charges a 2.1% fee for online processing. Fuck them. It should be a discount for online logistics, which have significantly reduced overhead.
• Even if it's Elavon, you need to front that.
• For number systems, the exponents always increase from 0 to n-1. This is known.
• For binary, the base is 2, so the digits in the number (the constants before the base-exponents) can only be those 2:
• [0, 1]
• For decimal, the base is 10, so the digits in the number (the constants before the base-exponents) can only be those 10:
• [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
• I had thought about the base/exponent before, but never the limited range of digit values.
• For previous-project questions, use STAR: Situation, Task, Action, Result.
• My Amazon interview is actually at the Santa Monica location, which is more convenient! The hiring managers can pass offers between sites after, if need be.
• Took a ton of "give an example of ..." notes. Moved gdrive docs around, cleaned.
• Database partitioning. Horizontal partitioning is when you keep the table the same and split groups of rows across different machines. Each is usually called a shard. Vertical partitioning is when you split columns off into a new table. Both improve performance, making it distributed (but more complex).
• Collections.deque is pronounced "deck" - it stands for double ended queue.
• Of course threads in a process have their own stacks but share heap. Separate processes get their own stack and heap.
• Disjoint sets. Another useful data structure. Usually implemented as trees. Two operations: find() and union().

• Promise. Async object that will return a value in the future.
• If you call an async function, the promise is returned immediately (synchronously) but in a pending state. Later it will become fulfilled or rejected and do something else.
• Can attach a callback to handle the return.
• .then() returns a promise.
• Functional vs OOP.
• Functional uses (wait for it) functions - pure, no shared state, no global mutation. It's based around actions, and is better if you expect your project to scale by adding actions.
• Object oriented is usually based classes that interact. They share. It's based around things, and is better if you expect your project to scale by adding things.
• Function composition. Could be decorators (chained functions) or simply meshing the results of multiple returns today. Straightforward.
• Class inheritance vs prototype inheritance.
• The class is a template, not an object. Gives you a template for your subclass.
• The prototype is an actual object. You create clones or slightly different children of it.
• General composition vs inheritance.
• Does B want ALL features of A? Inherit. Only want some? Compose.
• I generally agree with this: https://medium.com/inc./amazon-uses-a-secret-process-for-launching-new-ideas-and-it-will-transform-the-way-you-work-aec5c9121ae.
• Work backward from the customer, not forward from the product.
• Start with the problem and find the solution, don't create something and then check what it helps.
• Couldn't find a decent ebook download, so I ordered a hardcover for the first time in years. The man who solved the market, jim simons, rentech. Excited to read, but will probably wait until after interviews.
• NFC wild card games. Vikings and Seahawks advance.
• Divisional games: Ravens/Titans, Chiefs/Texans, Niners/Vikings, Packers/Seahawks.
• The overtime rules are so dumb. The percentage of a goalie blocking a pk in a shootout is about 30%. That's about the same ballpark as the probability of getting a touchdown on a drive. Imagine if the soccer game ended when the goalie blocked the first kick.
• Made 12 new powders.
• Caught up with Spencer. Slater's on the pier. Miss him! Bomb nashville chicken sandwich.
• Talked interviews, personal projects, SpaceX, software, life, investing.
• Rewriting the autotest ui and other frontends in python with transcrypt.
• Buying stock in a company is not directly financially enabling them, the stock pool is mostly set. It's just changing hands. Buying a vote in investors meetings. It does increase volume and other things that make the company seem more valuable.
• Certification got convoluted. Half is encryption, the other half is identification. They should be separate.
• Practice problems.
• I'm gold for problem solving. https://www.hackerrank.com/scoring.
1. https://www.hackerrank.com/challenges/friend-circle-queries/problem. Wrote a solution in 10min that passed half the tests. You can optimize further with disjoint sets, but I feel comfortable with my non-fancy solution.
2. https://www.hackerrank.com/challenges/maximum-xor/problem. Solved the brute force way, O(mn). To get it in linear time instead of quadratic, you have to use something called a Trie, which is basically a binary tree with 0s and 1s. https://en.wikipedia.org/wiki/Trie.
3. https://www.hackerrank.com/challenges/insert-a-node-at-a-specific-position-in-a-linked-list/problem. Standard linked list insert into position. I always have to think for a minute about the while loop. Manage your own index, keep track of prev/next, and make a few things conditional on index == 0. You can simplify it if you create an entirely new list, but the in-place version is better for space situations.
4. https://www.hackerrank.com/challenges/insert-a-node-into-a-sorted-doubly-linked-list/problem. I wrote the long-winded solution, which worked. HOWEVER. You can do these ll problems with recursion. It's way more concise, it's just a bit harder to conceptualize imo.
5. https://www.hackerrank.com/challenges/reverse-a-doubly-linked-list/problem. Same thing. Try to thing about the recursive solution. If it's too complicated, fall back to iterative.
6. https://www.hackerrank.com/challenges/find-the-merge-point-of-two-joined-linked-lists/problem. Looped through both. Quadratic solution, which passed all tests, but there is an O(n) algorithm on the discussion page. It's just less intuitive.
7. https://www.hackerrank.com/challenges/ctci-linked-list-cycle/problem. Check if there's a cycle in a linked list. Easy. The key here is knowing that you can store objects as keys in a hash table. Then simply loop through the ll and bookkeep them. If you ever already have one, there's a cycle. The hash lookup is constant time.
8. https://www.hackerrank.com/challenges/binary-search-tree-lowest-common-ancestor/problem. Tree problems make me think for a bit longer. Remember it's like a linked list, but you each node has two pointers instead of one. This yields two calls in recursion, or two expansions in iteration. This particular one could be solved with math, because you're given a binary search tree that's already balanced, but you could walk it all the same to find the answer.
9. https://www.hackerrank.com/challenges/ctci-is-binary-search-tree/problem. Walk through a BST. I solved it both iteratively and recursively. An additional note: a binary search tree is valid if the inorder traversal is sorted properly.
10. https://www.hackerrank.com/challenges/tree-huffman-decoding/problem. Very easy question. Solved it quickly, but the wording was overly complicated.
11. https://www.hackerrank.com/challenges/balanced-forest/problem. Didn't try. This problem is too long.
12. https://www.hackerrank.com/challenges/torque-and-development/problem. Build an adjacency dict of {node: {neighboring nodes}}. Then traverse it and group them.
13. https://www.hackerrank.com/challenges/find-the-nearest-clone/problem. For BFS, literally just pop from the from of the queue instead of the end of the stack. The graph problems have been longwinded for me.
14. https://www.hackerrank.com/challenges/ctci-fibonacci-numbers/problem. Recursive fibonacci. Easy as pie. Remember it's 2^n (exponential) without dp, and down to O(n) with memoization.
15. https://www.hackerrank.com/challenges/ctci-recursive-staircase/problem. The staircase problem with 1/2/3 hops. This is the same as fibonacci with 3 numbers in the sequences, and the 3 exit conditions in order are 1/2/4. Remember these get much faster with a cache. Check if n is in the cache, and only recurse if it's not.
16. https://www.hackerrank.com/challenges/recursive-digit-sum/problem. Easy. Recurse.
17. https://www.hackerrank.com/challenges/crossword-puzzle/problem. Thought about it for about 20min, but then didn't take the time to solve. A very straightforward problem, but tedious. This is like a simpler version of the 5squared puzzle I've solved (programmatically).
18. https://www.hackerrank.com/challenges/balanced-brackets/problem. Easy. Just run a stack for open/close. Few small corner cases.
19. https://www.hackerrank.com/challenges/ctci-queue-using-two-stacks/problem. Lol what a silly question. Just implement the methods of a queue using a python list, enqueue/dequeue/peek.
20. https://www.hackerrank.com/challenges/largest-rectangle/problem. Same as the leetcode one. Use two pointers coming in from both sides.
• 11 left. Will do tomorrow.
• XOR of two integers is just the XOR of the bit representations of the integers. In python ^ is the operator for XOR. For example, 2 ^ 1 = 3.
• In python it's set.add vs list.append.
• In python sets are implemented with hash tables, so lookup is basically the same as a dict (extremely fast).

• Practice problems.
1. https://www.hackerrank.com/challenges/ctci-bubble-sort/problem. Just implement a bubble sort. One loop through n-1, check if bigger than next, swap if so. Then simply wrap the whole thing in a blind loop to run n times, which guarantees it'll be sorted when finished.
2. https://www.hackerrank.com/challenges/mark-and-toys/problem. Basic sort.
3. https://www.hackerrank.com/challenges/ctci-comparator-sorting/problem. Write a comparator method of a class. Pass two objects, do whatever logic, then return -1 if obj1 goes before obj2 or 1 if opposite. Then you can pass the Class.comparator function as a key to a sort call, or whatever.
4. https://www.hackerrank.com/challenges/fraudulent-activity-notifications/problem. Tougher, but I really enjoyed this one. Success rate was low, in the 30s. Sliding window for median, remember you can access it by index (middle if n odd, (middle + middle-1) if n even). To remove/insert in the sliding window, use the bisect module for speed.
5. https://www.hackerrank.com/challenges/ctci-merge-sort/problem. Brainstormed this one for about 30m and then chose not to implement. The question is basically "write you own merge sort". O(nlogn). This allows you to count the inversions. No thanks.
6. https://www.hackerrank.com/challenges/ctci-making-anagrams/problem. Simple. My solution was cool.
7. https://www.hackerrank.com/challenges/alternating-characters/problem. The easy ones are easy.
8. https://www.hackerrank.com/challenges/sherlock-and-valid-string/problem. Little more complicated, but in syntax. Nothing algorithmically special here.
9. https://www.hackerrank.com/challenges/special-palindrome-again/problem. Tired of palindromes. I don't like these problems.
10. https://www.hackerrank.com/challenges/common-child/problem. Didn't care to do this one either. Don't like longest common subsequence problems. Remember the table of 1s and 0s, slowly sum as you move down and right.
11. https://www.hackerrank.com/challenges/minimum-absolute-difference-in-an-array/problem. Nice change of pace. GREEDY algorithms. If you find yourself facing a quadratic solution, just sort! See if that helps.
12. https://www.hackerrank.com/challenges/luck-balance/problem. Simple.
13. https://www.hackerrank.com/challenges/greedy-florist/problem. Solved, but dumb question. It's worded very ambiguously, and I had to modify the footer because the main function was completely missing a required argument (first time I've seen this on hackerrank).
14. https://www.hackerrank.com/challenges/angry-children/problem. Easy. Fast. Don't forget outside of this section; consider a sort!
15. https://www.hackerrank.com/challenges/reverse-shuffle-merge/problem. Not gonna touch this one. Not worth my time, since it's limited. Trying to understand the problem description alone took 10 minutes.
16. https://www.hackerrank.com/challenges/ctci-ice-cream-parlor/problem. Find a pair within 1d array, so iterate through it once, build a complement map.
17. https://www.hackerrank.com/challenges/swap-nodes-algo/problem. Didn't do. This question takes 30 minutes to read.
18. https://www.hackerrank.com/challenges/triple-sum/problem. One you have to think about, doesn't fit a standard pattern. Solve an easy case and you'll see you're best off approaching along a specific dimension (q).
19. https://www.hackerrank.com/challenges/minimum-time-required/problem. An interesting one as well. Basically implement your own binary search. Find best and worst case for lower and upper bounds, bisect them, calc the result, then shift one bound depending upon what the produced value is. Put this all in a loop `while lower_bound < upper_bound`.
20. https://www.hackerrank.com/challenges/maximum-subarray-sum/problem. <30% solve rate. Took a stab at it. Started with ~30m of whiteboarding. Reached the conclusion that the module of the sum is the modulo of (the sum of the modulos of the individuals). This is the key piece.
21. https://www.hackerrank.com/challenges/making-candies/problem. Not really a search problem, just walk through the simulation. My solution was the right complexity (passed 35/49 tests), just needed smaller optimizations here and there.
22. https://www.hackerrank.com/challenges/flipping-bits/problem. "Flipping bits" is just subtracting from the max size. So if you flip bits on a 32bit unsigned integer x, the result is 2^32-1-x.
23. https://www.hackerrank.com/challenges/ctci-big-o/problem. Check if a number is prime. To do so, iterate only over odd numbers (because even divides by 2), and only up to square root of x. Every pair of factors, if it exists, has one number below the sqrt and one above, so you only need to check up to the integer at floor(sqrt(x)).
• Only 30 problems left. The average difficulty is a little on the easier side, too. The hardest is like 40% pass rate. I'll try to do most tomorrow so that I can focus on the open-ended questions monday. Behavioral, design, etc.
• To switch two items in an array in python in one operation, just do:
• list[index1], list[index2] = list[index2], list[index1]
• Instead of doing quotient = math.floor(num/den) and rem = num % den, get both with quotient, rem = divmod(num, den).
• Oh shit. You can also get the quotient directly with // in py3. 11 // 2 results in 5. Don't need math.floor everywhere.
• If n is given for a problem to be 10^9, you probably need a linear solution. If 10^5, try to find an nlogn algorithm. If 10^4, quadratic is ok.
• Red-black trees are colored to preserve balance.
• Push, meditate.
• Ordered more preferred stock.
• AFC playoffs. Texans and Titans moving on.
• Supercontest. Disabled the emailer during the offseason.
• The machine was out of disk space, so I sshed in and ran docker system prune -af to free up 17.89GB. Remember to create the network and restart the nginx-proxy containers as well after this operation.

• Took the BMW online test. 8 questions. Each was a 1-3m video recording of my answer.
• All had unlimited time to think - I was kinda hoping they'd be restricted in time and in #takes (could shine more I guess).
• The whole test was java heavy.
• JDK/JRE/JVM. Generics.
• Creational design patterns. Prototype, factory, singleton.
• JEE APIs. JavaMail - like flask-mail. Persistence - like sqlalchemy. Security API - like flask-user and flask-admin.
• Join differences.
• I know these, just typing out for refresher:
• Inner. Normal, condition matches in both tables.
• Left. Return the full left table with matching records from the right, and null otherwise.
• Right. Return the full right table with matching records from the left, and null otherwise.
• Full. Return both left and right (basically stitch both tables together and fill null with nothing).
• Good summary: http://www.sql-join.com/sql-join-types.
• Flash will not be supported on chrome at the end of 2020. I'm still amazed Amazon's conference room requires it.
• Apple TV+, Disney+, Hulu, Showtime, HBO, Netflix, Peacock (NBC) coming soon.
• Smoked a pork shoulder. Handpressed tacos with maseca.
• Cooked a couple pounds of garbanzo beans.
• Turns out that the IR lamp in the terrarium isn't dead, I simply had the outlet disabled.
• Supercontest.
• Had to decide to implement an offseason explicitly or simply extend week 17 to last through the remainder of the year. The latter is much easier, so I did that. The former bleeds into g.current_week, current_season, and downstream into is_paid_user, leagues, and much more. It's more proper, but much more complicated and unnecessary at this point. If I need an offseason-specific view in the future, I can add this.
• Fixed and deployed.
• Practice problems.
• Whiteboard most of these. Transferred to computer to check syntax.
• Another thing above leetcode that's superior to hackerrank: it reports time and mem usage, as well as your percentile.
• https://www.hackerrank.com/challenges/counting-valleys/problem.
• https://www.hackerrank.com/challenges/jumping-on-the-clouds/problem.
• https://www.hackerrank.com/challenges/repeated-string/problem.
• https://www.hackerrank.com/challenges/2d-array/problem.
• https://www.hackerrank.com/challenges/ctci-array-left-rotation/problem.
• https://www.hackerrank.com/challenges/new-year-chaos/problem. This was started to get a little tougher. Write down a few simple cases, reason through the logic.
• https://www.hackerrank.com/challenges/minimum-swaps-2/problem. This one is dumb. Hacker rank has some pretty misleading/bad questions.
• https://www.hackerrank.com/challenges/crush/problem. 
• Sometimes a problem has two dimensions that you can iterate over, say N and M, and it’s worth putting an IF statement in your whole algorithm to dictate which you do.
• https://www.hackerrank.com/challenges/ctci-ransom-note/problem. Remember hash tables are faster lookup than arrays. If you’re going to access more than once later, it’s usually worth iterating over the array a single time creating a dict with word counts, and then accessing that later, rather than finding the word in the original array.
• https://www.hackerrank.com/challenges/two-strings/problem. Hash for lookup speed!
• https://www.hackerrank.com/challenges/sherlock-and-anagrams/problem. Dumb questions. Worded with zero clarity. Accepts a brute force O(n!) solution, but indicates nothing of such constraints.
• https://www.hackerrank.com/challenges/count-triplets-1/problem. Remember remember. For COUNTING problems, your gut should tell you to use a hashtable for fast lookup. I got a general solution that passed most the tests, but the final answer that passes all is VERY elegant. Was cool to read.
• https://www.hackerrank.com/challenges/frequency-queries/problem. Crushed it. Defaultdicts, tested with int, list, and dict. Kinda cool to have a defaultdict whose default is a nested dict.
• Went through the 4 basic "approach" videos as well: https://www.hackerrank.com/interview/interview-preparation-kit/tips-and-guidelines/videos.
• A factorial multiplies each integer's product, but in some cases for combinations you need to add them, like the sum of all integers from n to 1. This is called the binomial coefficient, and it's n(n+1)/2. It always yields an integer. This comes up in regular life way more often than you might think; pay attention for it.
• In python, collections.defaultdict(int) will make the default zero. Similarly, (list) will make the default an empty list. This is very useful to avoid the annoying "check if key exists, append if so, create if not" clauses.
• defaultdict is a little faster than Counter, as well.