Design and implement a data structure for Least Recently Used (LRU)
cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key
exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already
present. When the cache reached its capacity, it should invalidate the
least recently used item before inserting a new item.