Build It

Design it. Implement it. Prove it. 5 problems

Same problem, escalating constraints. Stage 1: naive solution. Stage 2: scaled. Stage 3: prove correctness with a conservation invariant — the senior filter. Each stage unlocks after you submit the previous.

5problems

1free

5languages

AI-graded

Easiest free problem · start here

Thread-Safe Wallet

free

In-memory wallet that evolves across four stages: basic ops → atomic transfer → thread-safe under contention → multi-currency. Each stage adds one constraint that the naive previous version violates.

concurrency·4 stages·90 min·C#, Go, Java, Kotlin, Python

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Durable Background Job Queue

pro

distributed·4 stages·110 min·C#, Go, Java, Kotlin, Python

Build enqueue, retry, leasing, dead-letter, and crash-recovery semantics for worker jobs.

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Idempotent Payment Processor

pro

payments·4 stages·100 min·C#, Go, Java, Kotlin, Python

Build a payment API that survives retries, concurrent duplicates, crashes, and event publication.

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Inventory Reservation Service

pro

concurrency·4 stages·100 min·C#, Go, Java, Kotlin, Python

Reserve, confirm, cancel, and expire inventory without overselling under contention.

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Notification Delivery Service

pro

api·4 stages·100 min·C#, Go, Java, Kotlin, Python

Build preference-aware, bounded, retryable notification delivery with cancellation and expiry.

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