[LintCode] LRU Cache 缓存器

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.

LeetCode上的原题，请参加我之前的博客LRU Cache。

class LRUCache{
public:
    // @param capacity, an integer
    LRUCache(int capacity) {
        size = capacity;
    }
    
    // @return an integer
    int get(int key) {
        auto it = m.find(key);
        if (it == m.end()) return -1;
        l.splice(l.begin(), l, it->second);
        return it->second->second;
    }

    // @param key, an integer
    // @param value, an integer
    // @return nothing
    void set(int key, int value) {
        auto it = m.find(key);
        if (it != m.end()) l.erase(it->second);
        l.push_front({key, value});
        m[key] = l.begin();
        if (m.size() > size) {
            int t = l.rbegin()->first;
            l.pop_back();
            m.erase(t);
        }
    }
private:
    int size;
    list<pair<int, int> > l;
    unordered_map<int, list<pair<int, int> >::iterator> m;
};