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.
原文：http://www.cnblogs.com/grandyang/p/4587511.html