This problem was asked by Google.
Implement an LFU (Least Frequently 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 frequently used item. If there is a tie, then the least recently used key should be removed.
get(key): gets the value at key. If no such key exists, return null.