▶ Open the course — runs in your browser, nothing to install.
An interactive data structures course for people starting from zero. Every structure is shown as a memory diagram first, then animated step by step, then written out in three languages side by side — because the hard part is rarely the syntax, it is not being able to see what the machine is doing.
Sister sites: AlgoAlgo (algorithms) and APIer (HTTP, REST and GraphQL).
The course home — 14 chapters, three languages side by side
Inside a chapter: memory diagram, live visualization, and code
| # | Chapter | What it covers |
|---|---|---|
| 00 | Prologue | Why structure matters · Big-O · the complexity cheat sheet |
| 01 | Array | Contiguous memory, the addressing formula, resizing, two pointers, sliding window |
| 02 | String | Immutability, the constant pool, encoding, concatenation costs, KMP |
| 03 | Linked list | References instead of adjacency, the dummy sentinel, fast/slow pointers |
| 04 | Stack | LIFO, the call stack, monotonic stacks |
| 05 | Queue | FIFO, circular buffers, deques, monotonic queues, BFS |
| 06 | Hash table | Hash functions, collisions, load factor, chaining vs. open addressing |
| 07 | Binary tree | Recursion, four traversals, top-down vs. bottom-up |
| 08 | BST | Ordering invariants, degeneration, AVL, red-black trees, B+ trees |
| 09 | Heap | Sift up/down, priority queues, top-K, two-heap median |
| 10 | Trie | Prefix trees, array vs. map children, autocomplete |
| 11 | Union-Find | Union by rank, path compression, the α(n) bound |
| 12 | Advanced | Composite structures — LRU, LFU, and friends |
| ✦ | Atlas | Signal-to-structure map: read a problem, pick the structure |
Each chapter follows the same rhythm: an intuition first, then an interactive visualization, then code in Java / Python / JavaScript, then the common mistakes, then a quiz. Progress is stored locally in the browser.
Requires Node 22 (an .nvmrc is included):
nvm use
npm install
npm run dev # http://localhost:3000Build with type checking: npm run build.
Next.js 15 (App Router) + TypeScript + React 19, plain CSS. There are no API routes, so the whole site prerenders to static pages.
Each chapter is one folder under app/ holding its page, its visualizations (viz.tsx) and
its own stylesheet, paired with a data file under lib/ for the problem sets.
Shared pieces: the frame-by-frame player in lib/stepper.tsx, the quiz component in
lib/quiz.tsx, progress tracking in lib/progress.tsx, and design tokens in
app/globals.css.
© 2026 Weiren Feng. All rights reserved. Published for reading and portfolio purposes; not licensed for reuse, modification, or redistribution.