Your data structure decision table
By the end of today you have a written decision table you can apply to any new problem, and you can defend a structure choice under pressure rather than reaching for a list by default.
YesterdayDays 13 to 21 covered cost, arrays, hashing, sets, stacks, queues, recursion, sorting, search, trees and classes.
TomorrowTomorrow begins Phase 3, where programs stop running alone and start talking to each other across a network.
Why this matters
Everything in Phase 2 was preparation for one skill: choosing correctly before you write anything. That choice is what interviewers probe and what determines whether a system survives growth.
- Choosing a structure
- Choosing a collection
- Sorting to enable
- Tradeoff reasoning
Learn it
70 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
45 minWrite your decision table as a real file you will keep: eight rows, each naming the situation, the structure, the cost, and what you give up. Then solve the ten-most-active-users problem in code and note where it would break if the stream were a hundred times larger.
Recall it
25 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
Free · sign inRate yourself against five named criteria per concept. Completion and mastery are tracked separately, and anything you rate shakily comes back automatically on a spaced schedule.
Recap
- 01Choosing a structure is choosing which operation you want to be cheap
- 02A nested loop over one collection is nearly always a signal
- 03Trees answer ranges; hash maps answer exact matches
- 04Complexity only matters where the input actually grows
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.