Implement concurrent structures and debug queue code

The coding rounds covered several implementation and debugging tasks: 1. Implement an LRU cache that supports `get(key)` and `put(key, value)` in constant time. Then explain how you would make the cache safe under concurrent access from multiple threads. 2. Implement a hash map from scratch with `put`, `get`, and `remove`. Then discuss or extend the design so that it works correctly when multiple threads read and write concurrently. 3. Implement a quadtree for two-dimensional points or rectangles, and support spatial query operations such as returning all items inside a query rectangle. 4. Review and debug a C++20 producer-consumer program built around a message queue. Identify correctness issues, race conditions, synchronization mistakes, and any performance or API-design problems.