Design a data structure that acts as a Least Recently Used (LRU) cache. Implement methods to get and put key-value pairs, evicting the least recently used entry when the cache is full.

Design a key-value store that supports storing values with timestamps. You should be able to retrieve the latest value for a key that's less than or equal to a given timestamp.

Determine if a given 9x9 Sudoku board is valid. Check if each row, column, and 3x3 subgrid contains the digits 1-9 without repetition.