LintCode_134 LRU Cache

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.

思路是用一個隊列和哈希表來實現：

每次get或者set的時候， 先將key壓入隊列尾部，然後在哈希表中對應的key的訪問次數+1；

如果滿了，進行while循環：查看隊列頭部的元素， 如果訪問次數>1，則說明後面還被訪問過， 故彈出， 如果訪問次數==1，則說明後面已經沒有被訪問過，所以彈出並且在哈希表中刪掉key。如此， 可以保證做少被訪問的元素被彈出（保證了訪問次數相同的情況下，先進先出）

class LRUCache{
public:
    // @param capacity, an integer
    LRUCache(int capacity) {
        // write your code here
        this->capacity = capacity;
    }
    
    // @return an integer
    int get(int key) {
        // write your code here
        if (map.find(key) != map.end()) {
            que.push(key);
            map[key].second++;
            return map[key].first;
        } else {
            return -1;
        }
    }

    // @param key, an integer
    // @param value, an integer
    // @return nothing
    void set(int key, int value) {
        // write your code here
        que.push(key);
        if (map.find(key) != map.end()) {
            map[key].first = value;
            map[key].second++;
        } else {
            if (map.size() < capacity) {
                map[key] = make_pair(value, 1);
            } else {
                int least_recently_key = que.front();
                while (map[least_recently_key].second > 1) {
                      map[least_recently_key].second--;
                      que.pop();
                      least_recently_key = que.front();
                }
                que.pop();
                //cout<<"erase "<<least_recently_key<<endl;
                map.erase(least_recently_key);
                map[key] = make_pair(value, 1);
            }
        }
    }
private:
    int capacity;
    unordered_map<int, pair<int, int>> map;
    queue<int> que;
};