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Problem library
High-level architecture and low-level object design, each with progressive hints, a written analysis, approach and worked solution.
Build a feed for millions of readers, from everyday posts to celebrity fan-out.
Design partitioning, bounded staleness and stampede control for a shared cache.
Build constant-time lookup and eviction with a hash map and a doubly linked list.
Serve ranked prefix suggestions while rebuilding indexes without downtime.
Turn uploads into adaptive streams with reliable transcoding and private playback.
Evaluate consistent rollouts with versioned rules and predictable outage defaults.
Maintain ordered scores with deduplicated updates, seasons and deterministic ties.
Track multi-device presence using heartbeats, expiry and stale-event protection.
Design edge caching, origin protection and reliable invalidation of published content.
Separate candidate generation, versioned ranking and real-time eligibility checks.
Evict by access frequency with an LRU tie-break and constant-time bookkeeping.
Combine exact expiry checks with generation-safe cleanup and bounded capacity.
Publish immutable content-addressed artifacts with safe tag updates and garbage collection.
Manage device sessions, refresh-token rotation and bounded revocation delays.
Resolve reproducible dependency graphs against immutable registry snapshots.
Schedule isolated build actions using complete input digests and trusted cache publication.
Evaluate bounded relationship graphs with revision-aware caching and revocation semantics.
Derive current access from independent grants, expirations and revocations with revision-aware checks.
Fetch immutable content chunks from untrusted peers with trusted manifests and origin fallback.
Sell seats for popular events without double-booking, even when millions arrive at once.
Count a firehose of view events and serve the most-viewed items for the last hour, day and month, fast and nearly in real time.
Serve track metadata and audio at scale while play events feed royalties and recommendations.
Answer route and ETA queries over a road graph while live traffic reports keep speeds fresh.
Rank posts by votes and age for a read-heavy front page while vote counts settle asynchronously.