The Must-Solve 150.
Every question is a real implementation, debugging or architecture task. Work through the levels in order, or jump straight to what your next interview needs.
10 of 150 questions
Hard
50 questions, 45–90 minutes each
Senior-level correctness, architecture and production constraints.
Frontend system design
Design the frontends of WhatsApp, Docs, YouTube, Sheets and more.
- 131Design WhatsApp Web: real-time messaging, ordering, offline state and reconciliation
- 132Design Google Docs: collaborative editing with CRDT or OT, cursors, and conflict resolution
- 133Design YouTube frontend: video playback, adaptive streaming, and feed virtualization
- 134Design Instagram/Pinterest Explore: infinite masonry grid, caching, and image optimization
- 135Design Google Sheets: virtualized cells, formulas, dependency graphs, and editing
- 136Design Amazon/Flipkart checkout: pricing, cart synchronization, payments, and failure recovery
- 137Design an autocomplete system serving millions of users with caching and request cancellationFree
- 138Design a real-time dashboard with WebSocket subscriptions, reconnects, and high-volume updates
- 139Design a large enterprise React application with micro-frontends, permissions, and deployment strategies
- 140Design Google Maps/Uber live tracking: map rendering, location updates, smoothing, and state synchronization