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

C++：

class LRUCache {
public:
    struct CacheNode{
        int key;
        int value;
        CacheNode(int k,int v):key(k),value(v){}
    };
    
    LRUCache(int capacity) {
        this->capacity = capacity;
    }
    
    int get(int key) {
        if(cachemap.find(key)==cachemap.end())
            return -1;
        //将当前访问的节点移动到列表头部，并且更新map中该节点的地址（索引）
        cachelist.splice(cachelist.begin(), cachelist, cachemap[key]);
        cachemap[key] = cachelist.begin();
        return cachemap[key]->value;
    }
    
    void put(int key, int value) {
        if(cachemap.find(key)!=cachemap.end()){
            //更新节点的值，把当前访问的节点移到列表头部，并且更新map中该节点的地址
            cachemap[key]->value = value;
            cachelist.splice(cachelist.begin(), cachelist, cachemap[key]);
            cachemap[key] = cachelist.begin();
        }
        else{
            if(cachelist.size()==capacity){//删除尾部最少访问的
                cachemap.erase(cachelist.back().key);
                cachelist.pop_back();
            }
            cachelist.push_front(CacheNode(key, value));
            cachemap[key] = cachelist.begin();
        }      
    }
private:
    //list用于存储当前容量的内存页，map为便于O(1)内查询到list中的某个页
    list<CacheNode> cachelist;
    unordered_map<int,list<CacheNode>::iterator> cachemap;
    int capacity;
};

/**
 * 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);
 */