How to use this

The goal isn’t to memorise questions. It’s to be fluent in the small set of ideas that top-tier interviews keep testing. When you can recognise the shape of a problem, you can rebuild the solution under pressure, in any language.

How every page is laid out

1. TL;DR: the idea in two sentences.
2. The idea in plain words: an analogy and the core insight.
3. See it: an interactive visualization. Change the input, step forward and back, and watch the invariant hold.
4. When to reach for it: the phrases in a problem statement that should make you think of it.
5. How it works: the template, then code in Python, TypeScript and C#. Your language choice is remembered across pages.
6. What it costs: time and space, and why.
7. Variations, pitfalls, and what to say in the interview.
8. Check yourself: short quizzes that test the reasoning, not recall.

A study loop that works

1. Read the page and play with the visualization until you can predict the next step before pressing ▶.
2. Close the page. Re-write the template from memory in your strongest language.
3. Do one of the “practice on your own” exercises, then a Sandbox problem from the same stage.
4. If you get stuck in a Sandbox round, the interviewer’s hints point you back to the right page here.

No spoilers

The code samples here are classic textbook exercises. They are deliberately not the Sandbox’s problems. Pages also never say which Sandbox problem uses which concept, because recognising that is part of the test.