See every step, and why
Synced pseudo-code, live variables and plain-English narration. Scrub back to any moment.
Today
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NewPredict mode: guess the next step, then check
Watch algorithms think.
Step through sorting, graphs, trees, dynamic programming and machine learning, forwards and backwards, with the code and the reason for every move. Then predict the next step yourself.
SortingBubble sort
step 21 / 37- 4
- 3
- 9
Compare neighbours 4 and 3.
Predict
Are 4 and 3 out of order?
- step-through visualisers
- 101
- hands-on lessons
- 70
- interview problems with real tests
- 91
- Python, JavaScript and TypeScript in your browser; Java, C++ and Go on our server
- 6 languages
Watching is the start. Predicting is where it sticks.
Every lesson moves you from seeing an idea, to calling its next move, to writing it yourself.
Call the next move
The player pauses at each decision and asks what happens next. Your answers feed your mastery.
Write it, watch your code run
Your code drives the same visualiser. When a test fails, you see exactly where it goes wrong.
From Big-O to Raft
- 7 visualisers · 5 conceptsData structures
- 14 visualisers · 7 conceptsSorting & searching
- 9 visualisers · 8 conceptsTrees & hashing
- 17 visualisers · 9 conceptsGraphs & strings
- 17 visualisers · 9 conceptsPatterns & design
- 25 visualisers · 21 conceptsMath & machine learning
- 12 visualisers · 11 conceptsSystems
- 5 case studies on a live traffic simulatorSystem design lab
A plan that skips what you know. Reviews that stop you forgetting.
Pick a goal and the hours you have. A five-minute placement check drops what you already know, and your plan re-orders itself as you learn. Problems and quiz questions come back just before you’d forget them.
- Goals for coding interviews, data structures and algorithms, machine learning foundations or deep learning and LLMs
- Works offline once a page has loaded
- Progress syncs when you sign in