LRU Cache is one of the most-asked design problems across all tech companies, and Amazon is no exception. DynamoDB and ElastiCache both use LRU eviction internally.
You'll learn to combine a hash map with a doubly linked list to get for both get and put. The hash map handles lookups, and the linked list maintains usage order. This problem also trains you to think about sentinel nodes (dummy head/tail) that simplify edge cases. If you can implement LRU Cache cleanly, you've proven you can design multi-structure systems.