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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.
Deliver ordered conversations across devices, reconnects and offline recipients.
Match riders with nearby drivers while handling stale locations and acceptance races.
Maintain ordered scores with deduplicated updates, seasons and deterministic ties.
Track multi-device presence using heartbeats, expiry and stale-event protection.
Accept one logical vote per participant and publish recoverable live aggregates.
Track device commands through reconnects, deduplication and ambiguous acknowledgements.
Run authoritative room state with ordered inputs, reconnects and ownership fencing.
Synchronize immutable file versions with cursors, tombstones and explicit conflicts.
Share document revisions with optimistic concurrency, resumable history and access revocation.
Serialize bids and deterministic closing under retries and delayed worker execution.
Form compatible game rosters while balancing skill, latency and queue wait time.
Assign support tickets by skills and capacity with aging, explicit ownership and reassignment.
Separate meeting admission, signaling and media forwarding with reconnectable participant sessions.
Derive enter/exit alerts from ordered location observations with uncertainty and hysteresis.
Let many people edit the same document at once, see each other's changes in real time, and never lose a keystroke.
Answer route and ETA queries over a road graph while live traffic reports keep speeds fresh.
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