LeetCode OJ：LRU Cache（最近使用缓存）

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.

最近使用缓存的问题，看到key和value感觉可能就要使用map了。用一个list维护一个cache，然后map指向相应cache的每个节点。

注意这里的最近使用，不管是get还是set都属于最近使用的范畴，代码如下所示：

 class LRUCache{
 struct cacheNode{
     int key;
     int val;
     cacheNode(int k, int v):
     key(k), val(v){}
 };
 public:
     LRUCache(int capacity) {
         size = capacity;
     }

     int get(int key) {
         if(cacheMap.find(key) != cacheMap.end()){
             auto it = cacheMap[key];
             cacheList.splice(cacheList.begin(), cacheList, it);
             cacheMap[key] = cacheList.begin();
             return cacheList.begin()->val;
         }else
             return -;
     }

     void set(int key, int value) {
         if(cacheMap.find(key) != cacheMap.end()){//list中存在该元素
             auto it = cacheMap[key];
             cacheList.splice(cacheList.begin(), cacheList, it);
             cacheMap[key] == cacheList.begin();
             cacheList.begin()->val = value;
         }else{                                  //list中不存在该元素
             if(cacheList.size() == this->size){
                 cacheMap.erase(cacheList.back().key);
                 cacheList.pop_back();
             }
             cacheList.push_front(cacheNode(key, value));
             cacheMap[key] = cacheList.begin();
         }
     }
 private:
     int size;
     list<cacheNode> cacheList;
     unordered_map<int , list<cacheNode>::iterator> cacheMap;
 };