LeetCode Q:146 LRU Cache (hard)

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.

Round 2 solution:
The key here is to keep a list iterator in hash map. We save a pair in list because we need know what key it is when we remove a value from list. So we can remove this key from hash map.