# 146. LRU Cache

Design and implement a data structure for [Least Recently Used (LRU) cache](https://en.wikipedia.org/wiki/Cache_replacement_policies#LRU). 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.

**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
```

```cpp
class LRUCache {
public:
    LRUCache(int capacity) : cap(capacity) {
        m.reserve(cap);
    }
    
    int get(int key) { // time: O(1)
        if (!m.count(key)) return -1;
        int value = m[key]->second;
        l.erase(m[key]);
        l.push_front({key, value});
        m[key] = l.begin(); // update the iterator in map
        return value;
    }
    
    void put(int key, int value) { // time: O(1)
        if (m.count(key)) l.erase(m[key]);
        l.push_front({key, value});
        m[key] = l.begin(); // update the iterator in map
        if (m.size() > cap) {
            int delete_key = l.rbegin()->first;
            l.pop_back();
            m.erase(delete_key);
        }
    }
private:
    int cap; // capacity
    list<pair<int, int> > l; // pair<key, value>
    unordered_map<int, list<pair<int, int> >::iterator> m; // <key, node in list>
};
```

```cpp
class LRUCache {
public:
    LRUCache(int capacity) : cap(capacity) {
        m.reserve(cap);
    }
    
    int get(int key) {
        auto it = m.find(key);
        if (it == m.end()) return -1;
        int value = it->second->second; // it->second means iterator::list<pair<int, int> >
        l.erase(it->second);
        l.push_front({key, value});
        m[key] = l.begin(); // update the key->iterator map
        return value;
    }
    
    void put(int key, int value) {
        auto it = m.find(key);
        if (it != m.end()) l.erase(m[key]);
        l.push_front({key, value});
        m[key] = l.begin();
        if (m.size() > cap) {
            int delete_key = l.rbegin()->first;
            l.pop_back();
            m.erase(delete_key);
        }
    }
private:
    int cap; // capacity
    list<pair<int, int> > l; // pair<key, value>
    unordered_map<int, list<pair<int, int> >::iterator > m;
};

```
