Implement an LRU cache with O(1) get/put, then add TTL.

Combine a hash map with a doubly linked recency list for O(1) get and put. Move hits to the front, evict from the tail, and integrate TTL with expiry checks or a heap; concurrency needs a clear lock or sharding policy.

Test empty input, one-element input, duplicates, boundary indices, invalid states, and the largest allowed size; state time and space complexity.