You’ve done LeetCode. Real interviews still break you.
LeetCode prepares you for algorithms. Gronex prepares you for real backend coding rounds. Practice production-style backend problems in real GitHub-like repositories — debug services, fix business logic, pass tests, and compare with real solutions, in Java, Python, and C++.
Built for SDE1+ and SDE2 backend interviews.
$ ./verify.sh
PASS OrderServiceTest.createsOrder
PASS OrderServiceTest.appliesDiscount
FAIL OrderServiceTest.processesConcurrentCheckouts
expected inventory: 0
actual inventory: -7 ← race condition
2 passed, 1 failed
Find the bug. Fix it. Make it green.Why this is different
Practice reading existing repository structure, contracts, and failing states — the rounds LeetCode and HackerRank don’t cover — instead of only writing fresh code.
Who it is for
Mid-level and aspiring mid-level backend engineers who are interviewing now, plus anyone leveling up debugging, concurrency, and code-reading.
How it works
Open a free challenge, download the watermarked starter repo, fix the failing tests locally with the bundled verify.sh, then compare against a reviewed diff.
Five challenge types, one real repo each
Every challenge is a production-style repository with failing tests — the format changes, the realism doesn’t.
Repository Implementation
Implement and fix real backend features — bookings, payments, wallets — inside an existing codebase.
Concurrency
Races, deadlocks, and locking bugs that only show up when requests collide.
Architecture Extension
NewTake a working service and extend it to support a new feature under interview constraints — the round startups actually give you.
API Contract Debugging
NewHit an API, compare expected vs actual responses — wrong statuses, broken idempotency, leaky auth — and fix the handler until the contract holds.
Incident Debugging
NewA service crashed in production. Read the logs, find the root cause, and fix it until it comes back green.
Learn how real systems are built, one annotated diagram at a time.
Study reference architectures with detailed, full-size diagrams and clear written walkthroughs — from core building blocks to trade-offs interviewers actually probe. Included with Gronex Pro, with free guides to get you started.
Problem formats
Representative interview tasks
Launch scope