#### 146. LRU Cache ####

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

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
put(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.

Follow up:
Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

手写##LRU Cache##的原理实现，使用双端链表：

class LRUCache {
public:
    LRUCache(int capacity) {
        size = capacity; //init the LRUCache object with capacity
    }
    
    int get(int key) {
        if(table.find(key)==table.end())
            return -1;  // no corresponding data
        
        auto it = table.find(key);
        lru.splice(lru.begin(), lru, table[key]); // move the data to the front of lru list
        table[key] = lru.begin(); // change the iterator of corresponding  data
        return (*table[key]).second;
    }
    
    void put(int key, int value) {
        if(table.find(key)==table.end())
        {
            if(lru.size()<size) // not full
            {
                lru.push_front(make_pair(key, value));
                table[key] = lru.begin(); // add into the header of the lru list
            }
            else
            { // full
                table.erase(lru.back().first);
                lru.pop_back();
                lru.push_front(make_pair(key, value));
                table[key] = lru.begin();
            }
        }
        else
        {
            (*table[key]).second = value;
            lru.splice(lru.begin(), lru, table[key]);
            table[key] = lru.begin();
        }  // already exists
    }
private:
    int size; //the capacity of lru
    unordered_map<int,list<pair<int, int>>::iterator> table; // (key, the iterator in lru)
    list<pair<int, int>> lru; // the data list, each element including key and value;
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */