146. LRU Cache

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