For developers
Four tracks that cover the whole loop — the concepts, the algorithm problems, the SQL, and the system design round. Every problem runs against real test cases before it reaches you.
Not sure where to start? Coding is the most popular track
Preparing for interviews? Try a mock interviewThey are independent — but run in order, they cover a full interview loop from first principles to a timed round.
Algo Coach opens every topic with a plain-English explanation and an animation you can step through — so you meet a problem already knowing what shape the answer takes.
Problems are generated on demand and executed against their own test cases before you ever see them. Filter by topic and difficulty, code in the browser, get instant feedback.
Sit a timed mock interview or a graded system design round. Three problems, 45 minutes, no hints until it ends — then a stage-by-stage read on where you actually stand.
No install. Open a problem and start writing code now — sign in free when you're ready to run it.
FAQ
If you are preparing for interviews and already know the basics, start with Coding — it is the most popular track and needs no setup. If the concepts themselves are shaky, start with Algo Coach instead: it teaches each data structure visually before you write any code. SQL and System Design are independent tracks you can pick up at any point.
No. Every track runs in the browser — the code editor, the test runner, and the SQL query engine all execute against real datasets without a local setup. You can start solving on any machine you can open a browser on.
Reading problems and drafting code needs no signup. Running and submitting code takes a free account, which also saves your progress, streak, and solved history across devices. Generating brand-new AI problems, mock interviews, and graded system design rounds draw from your token balance, which every account starts with.
Every coding and SQL problem is executed against its own test cases before it is ever shown to you — a generated problem that does not pass its own reference solution never reaches the library. Problems that fail verification are flagged, not quietly published.
Three things: problems are generated on demand so you are not re-solving a fixed list you have already memorised; Algo Coach teaches the concept visually before the problem instead of only after you fail one; and System Design gets you a graded, stage-by-stage interview round rather than an article to read.
Yes. Filter Coding by topic and difficulty to drill exactly the patterns a company screens for, then sit a timed mock interview — three problems, 45 minutes, no hints until it ends — to check where you actually stand under pressure.