# LRU Cache

## Description

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:**

```java
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
```

## **Code**

```java
public class LRUCache {

    class DLinkedNode {
        int key;
        int value;
        DLinkedNode pre;
        DLinkedNode next;
    }

    private void addNode(DLinkedNode node) {
        node.pre = head;
        node.next = head.next;
        head.next.pre = node;
        head.next = node;
    }

    private void removeNode(DLinkedNode node) {
        DLinkedNode pre = node.pre;
        DLinkedNode next = node.next;

        pre.next = next;
        next.pre = pre;
    }

    private void moveToHead(DLinkedNode node) {
        this.removeNode(node);
        this.addNode(node);
    }

    private DLinkedNode popTail() {
        DLinkedNode res = tail.pre;
        this.removeNode(res);
        return res;
    }

    private HashMap<Integer, DLinkedNode> cache;
    private int count;
    private int capacity;
    private DLinkedNode head, tail;

    public LRUCache(int capacity) {
        cache = new HashMap<>();
        this.count = 0;
        this.capacity = capacity;
        head = new DLinkedNode();
        head.pre = null;

        tail = new DLinkedNode();
        tail.next = null;

        head.next = tail;
        tail.pre = head;
    }

    public int get(int key) {
        if (!cache.containsKey(key)) {
            return -1;
        } else {
            DLinkedNode node = cache.get(key);
            this.moveToHead(node);
            return node.value;
        }
    }

    public void put(int key, int value) {
        if (!cache.containsKey(key)) {
            DLinkedNode newNode = new DLinkedNode();
            newNode.key = key;
            newNode.value = value;

            cache.put(key, newNode);
            this.addNode(newNode);

            ++count;

            if (count > capacity) {
                DLinkedNode tail = this.popTail();
                cache.remove(tail.key);
                --count;
            }
        } else {
            DLinkedNode node = cache.get(key);
            node.value = value;
            this.moveToHead(node);
        }
    }

}


/**
 * 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);
 */
```
