Trees, and why every database index is one
By the end of today you can traverse a tree recursively, explain why a balanced tree gives logarithmic lookup, and say what goes wrong when a tree becomes unbalanced.
YesterdayOn Day 19 you halved a search range each step. A tree stores that halving permanently instead of recomputing it, and Day 18's recursion is how you walk it.
TomorrowTomorrow you meet objects and classes, and start bundling data with the operations that belong to it.
Why this matters
Trees are the structure underneath file systems, JSON, the DOM, and database indexes. When Day 43 explains a B-tree index, you will already know why the shape matters.
- Trees
- Tree traversal
- Balance
- Trees in real systems
Learn it
75 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.
4 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 minImplement a binary search tree with insert and search using a simple node class. Add all three traversals. Insert ten random values and confirm in-order traversal returns them sorted. Then insert ten sorted values into a fresh tree, measure its height, and explain what happened.
Recall it
20 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
- 01A tree is nodes with children, one root and no cycles
- 02The search invariant is what lets you discard half the tree at each step
- 03Sorted insertion degenerates a tree into a list, which is why balancing exists
- 04Trees keep order, so they answer ranges; hash maps do not
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.