Practise coding interviews in your own editor.

• a.

  Your editor, real toolchains

  Each language is an ordinary project, built with uv, pnpm, Gradle, Go modules or Cargo, that your editor and its language server open directly. Breakpoints work too: cstudy test --debug waits for your debugger.

• b.

  More than algorithms

  Data structures and algorithms, practical problems that grow over levels, backend and service projects, low-level design, concurrency and debugging, security and AI engineering problems, plus system design and code review prompts that you grade yourself with a rubric.

• c.

  Optimality, not only correctness

  cstudy bench measures how your solution grows with the input size and compares it with the reference approaches, so you know whether a passing solution is also the right one.

• d.

  Spaced repetition

  cstudy done schedules the next review from how the attempt went, and cstudy next resumes what you have open, brings back what is due or picks a new problem from your weakest pattern.

New to a topic?With cstudy learn, read short guides that teach one idea before you practise it, with a worked example and diagrams, and follow a path on hashing that puts guides and problems in order. The local page shows them too. Guides and paths

1. 1

   Pick a problem

   cstudy next resumes an attempt you have open, or suggests a review that is due, or a new problem from the pattern where you are weakest, and prints the command that starts it.

   $ cstudy next
   new     koko-eating-bananas  (medium · binary-search)  weakest pattern: binary-search (0.75)
   start   cstudy start koko-eating-bananas --lang kotlin

2. 2

   Start it

   cstudy start writes a stub with the right signature and a short summary of the problem, renders the statement and opens both in your editor. The attempt’s clock starts here.

   $ cstudy start two-sum --lang kotlin
   two-sum · easy · arrays-hashing · kotlin
   stub       jvm/kotlin/src/main/kotlin/practice/two_sum/Solution.kt
   statement  .statements/two-sum.md
   next       cstudy test --watch

3. 3

   Solve it in your editor

   Your solutions live in a workspace of your own, next to your notes and progress. Open it in VS Code, IntelliJ IDEA, Neovim or anything else: completion, diagnostics and debugging work as in any project.

   ~/cstudy-workspace
     python/        pyproject.toml, uv.lock, practice/
     typescript/    package.json, tsconfig.json, practice/
     jvm/           Gradle wrapper, java/ and kotlin/ projects
     go/            go.mod, practice/
     rust/          Cargo.toml, practice/
     projects/      one folder per project problem
     notes/         your answers to system design and code review prompts
     sets.yaml      your own lists of problems
     progress.db    your attempts and reviews, in SQLite

4. 4

   Test on every save

   cstudy test --watch runs your code against the cases each time you save. Visible cases are always listed; a hidden case that fails shows its input, the expected answer and yours. Time limits are off unless you ask for them.

   $ cstudy test --time-limit
   two-sum · kotlin · 16 cases (2 visible, 13 hidden, 1 generated) · time limit 5x · timer 12:31 / 25:00
   build  1.8s (gradle, warm)
   
   PASS     ex1   0.04ms
   PASS     ex2   0.03ms
   FAIL     h1    hidden
            input     nums=[-3, 8, 0, 3]  target=0
            expected  [0, 3]  (unordered)
            actual    [2, 2]
            note      negatives and zero
   TLE      g1    hidden · 412ms > limit 95ms (5x hash-map, calibrated)
   PASS     12 hidden cases
   
   14/16 passed · 1 failed · 1 tle · total 2.6s

5. 5

   Finish, and come back when it is due

   Rate the attempt easy, ok or hard. Hints, a revealed solution or a blown time box lower the grade, and the grade sets the next review.

   $ cstudy done --rating ok
   two-sum · kotlin · passed · 18m 40s · rating ok · hints 0
   next review  2026-10-27 (in 3 days) · ease 2.50

Stuck?cstudy hint reveals one hint at a time, and cstudy solution shows the reference approaches with their explanations. During an unsolved attempt it asks first, because revealing grades the attempt as again. To stop without finishing, cstudy abort ends the attempt, keeps your files and leaves your review schedule as it was.

• Python272 problemsuv and Python 3.13
• TypeScript295 problemsNode 24 and pnpm
• Java250 problemsJDK 21 to 26, Gradle wrapper
• Kotlin250 problemsJDK 21 to 26, Gradle wrapper
• Go247 problemsGo 1.25 or newer
• Rust244 problemsRust 1.80 or newer, via rustup

236 of the 320 problems have reference solutions in all 6 languages. The others are project problems, each written for one language; problems that a language cannot express or whose bug exists only in some languages; and security and AI engineering problems, most of them in Python and TypeScript.