LeetCode 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.

The cache is initialized with a positive capacity.

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:
    LRUCache(int capacity) {
        cap = capacity;
    }
    
    int get(int key) {
        auto it = m.find(key);
        if(it == m.end())
            return -1;
        else
        {
            l.splice(l.begin(),l,it->second);
            return it->second->second;
        }
    }
    
    void put(int key, int value) {
        auto it = m.find(key);
        if(it != m.end())
        {
            l.erase(it->second);
        }
        l.push_front(make_pair(key,value));
        m[key] = l.begin();
        if(m.size() > cap)
        {
            int k = l.rbegin()->first;
            l.pop_back();
            m.erase(k);
        }
    }
private:
    int cap;
    list<pair<int,int>> l;
    unordered_map<int,list<pair<int,int>>::iterator> m;
};