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.

Follow up:Could you do both operations in O(1) time complexity?

The problem can be solved with a hashtable that keeps track of the keys and its values in the double linked list.

class Node { 
    int key; 
    int value; 
    Node pre; 
    Node next;

    public Node(int key, int value) {
        this.key = key;
        this.value = value;
    }
}
public class LRUCache {

    HashMap<Integer, Node> map; 
    int capicity, count; 
    Node head, tail;
    
    public LRUCache(int capacity) {
        this.capicity = capacity;
        map = new HashMap<>();
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.next = tail;
        tail.pre = head;
        head.pre = null;
        tail.next = null;
        count = 0;
    }
    
    public void deleteNode(Node node) {
        node.pre.next = node.next;
        node.next.pre = node.pre;
    }
    
    public void addToHead(Node node) {
        node.next = head.next;
        node.next.pre = node;
        node.pre = head;
        head.next = node;
    }
    
    public int get(int key) {
        if (map.get(key) != null) {
            Node node = map.get(key);
            int result = node.value;
            deleteNode(node);
            addToHead(node);
            return result;
        }
        return -1;
    }
    
    public void set(int key, int value) {
        if (map.get(key) != null) {
            Node node = map.get(key);
            node.value = value;
            deleteNode(node);
            addToHead(node);
        } else {
            Node node = new Node(key, value);
            map.put(key, node);
            if (count < capicity) {
                count++;
                addToHead(node);
            } else {
                map.remove(tail.pre.key);
                deleteNode(tail.pre);
                addToHead(node);
            }
        }
    }
}