LRU Cache 

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.

参考别人的代码。Node数据结构，dll中含有cache的基本信息头和尾的位置。LRU的最近最常使用的策略。所以该策略的是first要最近用过的，last则是要淘汰的。