Learn the pattern / prove the reasoning
Know why it works.
Then try it without hints.
Understand the state changes. Predict unseen inputs. Explain the invariant.
Return to the patterns you missed instead of just collecting solved questions.
81 original exercises, 81 interactive traces. A focused foundation—not a complete interview curriculum or a promise of getting hired.
Your roadmap
10 topics / 81 worked questions / 81 animationsArrows suggest a learning order, not strict prerequisites. Pick any topic to begin.
The same five values appear in each box. Each representation gives them different rules.
Arrays & hash maps
Look up what you have already seen.
Array / each value has a position16 questions / open notes 02 / topicStrings & windows
Keep just the part of a string you need.
Text / window "1325" has no repeats10 questions / open notes 03 / topicStacks & parsing
Pause a calculation. Resume it later.
Stack / last in, first out3 questions / open notes 04 / topicTrees & traversal
Explore a hierarchy without losing your place.
Tree / parent and child links14 questions / open notes 05 / topicGraphs & paths
Explore connections. Choose a better route.
Graph / an extra link makes a cycle10 questions / open notes 06 / topicHeaps & priority
Take the most useful next item.
Min-heap / smallest at the root3 questions / open notesThese are examples of representation, not required conversions between topics.
Complete the other patterns
All 75 original references →The original roadmap is unchanged. Bits, dynamic programming, linked lists and intervals have their own topic menus.
Prefer to start with an example?
Every question has a walkthrough.
Move one step at a time. See the state change, then read the line that caused it.