LRU（双向链表和hash表）

class LRUCache {
public:
    struct Node
    {
        int    key;
        int    val;
        Node( int _k, int _v ) : key( _k ), val( _v )
        {
        }
    };
    list<Node>                    l;
    unordered_map<int, list<Node>::iterator>    mp;
    int                        size;
    int                        capacity;
    LRUCache( int _capacity ) : capacity( _capacity ), size( 0 )
    {
    }


    int get( int key )
    {
        auto it = mp.find( key );
        if ( it == mp.end() )
            return(-1);
        else{
            int ans = it->second->val;
            l.erase( it->second );
            l.push_front( Node( key, ans ) );
            it->second = l.begin();
            return(ans);
        }
    }


    void put( int key, int value )
    {
        auto it = mp.find( key );
        if ( it != mp.end() )
        {
            l.erase( it->second );
            l.push_front( Node( key, value ) );
            it->second = l.begin();
        }else  {
            if ( size < capacity )
            {
                l.push_front( Node( key, value ) );
                mp.insert( { key, l.begin() } );
                size++;
            }else  {
                Node node = l.back();
                l.pop_back();
                auto rm = mp.find( node.key );
                mp.erase( rm );
                l.push_front( Node( key, value ) );
                mp.insert( { key, l.begin() } );
            }
        }
    }
};