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?

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
public class LRUCache {

    private class Node {
        int key, value;
        Node pre, next;
        Node(int key, int value) {
            this.key = key;
            this.value = value;
            pre = null;
            next = null;
        }
    }
    
    int capacity;
    HashMap<Integer, Node> map;
    Node head, tail;
    
    public LRUCache(int capacity) {
        this.capacity = capacity;
        map = new HashMap<>();
        head = new Node(-1, -1);
        tail = new Node(-1, -1);
        head.next = tail;
        tail.pre = head;
    }
    
    public int get(int key) {
        if (!map.containsKey(key)) {
            return -1;
        }
        Node node = map.get(key);
        remove(node);
        addToTail(node);
        return node.value;
    }
    
    public void put(int key, int value) {
        if (get(key) != -1) {
            map.get(key).value = value;
            return;
        }
        if (map.size() == capacity) {
            map.remove(head.next.key);
            head.next = head.next.next;
            head.next.pre = head;
        }
        Node newNode = new Node(key, value);
        map.put(key, newNode);
        addToTail(newNode);
    }
    
    private void addToTail(Node node) {
        node.next = tail;
        node.pre = tail.pre;
        tail.pre = node;
        node.pre.next = node;
    }
    
    private void remove(Node node) {
        node.pre.next = node.next;
        node.next.pre = node.pre;        
    }
}

/**
 * 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);
 */
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