146. LRU Cache.cpp

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









 class LRUCache {
    list<int> cache;
    unordered_map<int, pair<int, list<int>::iterator> > mp;
    int csize;
public:
    LRUCache(int capacity) {
        csize=capacity;
    }

    int get(int key) {
        if(mp.find(key)==mp.end()) return -1;
        cache.erase(mp[key].second);
        cache.push_front(key);
        mp[key].second=cache.begin();
        return mp[key].first;
    }

    void put(int key, int value) {
        if(mp.find(key)!=mp.end()){
            // key found
            cache.erase(mp[key].second);

        } else {
            // key not found
            if(csize==cache.size()){
                int last=cache.back();
                cache.pop_back();
                mp.erase(last);

            }
        }
        cache.push_front(key);
        mp[key]={value, cache.begin()};
    }
};

/**
 * 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);
 */