Welcome to PrepArena
Learn DSA the way interviews test it — by fixing broken code.
Real code runs in your browser against real tests. Hints when you are stuck, a clear explanation and diff when you are done, and spaced-repetition flashcards so the answers stick.
Daily goal
3 more to go
Bug Hunt
Hand-made levels that teach one pattern each, ending in a two-bug boss.
Endless
147 verified bugs injected into 20 classic algorithms. Build a streak.
Flashcards
179 cards in 16 decks, one per technology, scheduled with spaced repetition.
Online compiler
Write and run code in 18 languages — Python, Java, C++, SQL, JS and more. Share with a link.
+13 moreInterview questions
412 most-asked questions across 16 technologies, with answers, code and a practice mode.
Revision notes
14 full-stack topics to revise, in a handwritten style, plus System Design and DSA PDFs.
Whiteboard
Hand-drawn concept boards for every topic. Present them, annotate PDFs and export to PDF.
Mind map
Every technology, section and key idea on one zoomable map. Open a branch, search, or hide the details to test your recall.
Pick a technology
Interview questions, notes, flashcards and a whiteboard for each one.
Read the ticket
Each level describes the symptom and shows the test cases, like a real bug report.
Debug & run
Edit the code, add console.log, run the tests with Ctrl + Enter. Infinite loops are cut off safely.
Learn the why
See the explanation and a diff of the fix, earn XP, and move on to the next pattern.