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 -1;
    }
    
    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;
};