How Lessons & Problems Work

The demonstration and the cook-off

Every good cooking class has two different kinds of sessions. In a demonstration, the chef shows you a technique — explains why it works, walks through it step by step, lets you watch closely. In a cook-off, you're handed raw ingredients and a dish to make, with minimal hand-holding — you attempt it yourself, ask for a hint only when you're truly stuck, and compare your result to the chef's version afterward.

Concept lessons are the demonstrations. They teach a structure or technique itself, and follow a fixed arc:

1. Motivation — what problem exists that this thing solves; what breaks without it.
2. Mechanics — how it works, down to memory layout, with visualizations.
3. Operations and their costs — every operation's complexity, with the reasoning, not just the number.
4. Implementation — built from scratch in Python and TypeScript.
5. Trade-offs — when to reach for it, when its alternatives win.
6. Quiz — a short check that it stuck.

Problem lessons are the cook-offs. They apply what you've learned, one problem each, solve-first: statement and constraints → your attempt → progressive hints → the brute force and its cost → the key insight → the optimal solution → variants.

The interactive blocks

Lessons use a few interactive blocks. Here's each one, live.

Code tabs — implementations always come in both languages, the same dish cooked two different ways. Pick your primary language, but skim the other; seeing the same idea twice separates the idea from the syntax.

def contains_duplicate(nums: list[int]) -> bool:
    seen = set()
    for x in nums:
        if x in seen:
            return True
        seen.add(x)
    return False

Complexity boxes — the dish's nutrition label, standardized cost summaries, always with the why:

Reveals — the tasting notes you only read after you've actually cooked, used for hints and solutions in problem lessons. Genuinely try first; then open them in order.

Quizzes — the taste test, instant feedback with an explanation either way. Your answers are saved locally in your browser, nowhere else.

Progress

Opening a lesson marks it visited; the sidebar and header track how much of the course you've covered. Progress lives entirely in your browser's local storage — there are no accounts and nothing leaves your machine.

Next: how the code editor itself works — running your solution, reading test results, and what to do when you're stuck.