http://www.cnblogs.com/dolphin0520/p/3741519.html
Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations:getandset.
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.
