Implement an LRU (Least Recently Used) cache. It should be able to be initialized with a cache size n, and contain the following methods:
- set(key, value): sets key to value. If there are already n items in the cache and we are adding a new item, then it should also remove the least recently used item.
- get(key): gets the value at key. If no such key exists, return null.
Each operation should run in O(1) time.