> For the complete documentation index, see [llms.txt](https://soumyajit4419.gitbook.io/ds-algo/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://soumyajit4419.gitbook.io/ds-algo/hash-table/various-implementation-using-stl/11.-lru-cache-v.imp.md).

# 12. LRU Cache (V.IMP)

Design a data structure that follows the constraints of a [**Least Recently Used (LRU) cache**](https://en.wikipedia.org/wiki/Cache_replacement_policies#LRU).

Implement the `LRUCache` class:

* `LRUCache(int capacity)` Initialize the LRU cache with **positive** size `capacity`.
* `int get(int key)` Return the value of the `key` if the key exists, otherwise return `-1`.
* `void put(int key, int value)` Update the value of the `key` if the `key` exists. Otherwise, add the `key-value` pair to the cache. If the number of keys exceeds the `capacity` from this operation, **evict** the least recently used key.

**Example 1:**

```
Input
["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1);    // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2);    // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1);    // return -1 (not found)
lRUCache.get(3);    // return 3
lRUCache.get(4);    // return 4
```

## Solution: (Using Hashmap and linked list stl)

```cpp
class LRUCache
{
public:
    list<pair<int, int>> ll;
    unordered_map<int, list<pair<int, int>>::iterator> m;
    int cap, count;

    LRUCache(int capacity)
    {
        count = 0;
        cap = capacity;
    }

    int get(int key)
    {
        if (m.find(key) != m.end())
        {
            list<pair<int, int>>::iterator itr;
            itr = m[key];

            //Refresh Cache
            ll.push_front({key, (*itr).second});
            ll.erase(itr);
            m[key] = ll.begin();

            return ll.front().second;
        }

        return -1;
    }

    void put(int key, int value)
    {
        if (m.find(key) != m.end())
        {
            list<pair<int, int>>::iterator itr;
            itr = m[key];
            
            //Refresh Cache
            ll.push_front({key, value});
            m[key] = ll.begin();
            ll.erase(itr);
        }
        else
        {
            //Insert new value
            ll.push_front({key, value});
            m[key] = ll.begin();
            count++;
            
            //Check size and erase old
            if (count > cap)
            {
                m.erase(ll.back().first);
                ll.pop_back();
                count--;
            }
        }

        return;
    }
};
```

**Time Complexity: Get : O(1), Put : O(1)**