> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/linked-list/lru-cache.md).

# 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.

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
```

```java
class LRUCache {

    Node head = new Node(0, 0), tail = new Node(0, 0);
    HashMap<Integer, Node> map = new HashMap();
    int capacity;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        if (map.containsKey(key)) {
            Node node = map.get(key);
            remove(node);
            insert(node);
            return node.value;
        } else {
            return -1;
        }
    }

    public void put(int key, int value) {
        if (map.containsKey(key)) {
            remove(map.get(key));
        }
        if (map.size() == capacity) {
            remove(tail.prev);
        }
        insert(new Node(key, value));
    }

    private void remove(Node node) {
        map.remove(node.key);
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }

    private void insert(Node node) {
        map.put(node.key, node);
        Node headNext = head.next;
        head.next = node;
        node.prev = head;
        headNext.prev = node;
        node.next = headNext;
    }

    class Node {
        Node prev, next;
        int key, value;

        Node(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }
}
```