• [x] ### HashMap
• [x] ### Double Linked List

146.LRU Cache（HashMap + Double Linked List）

Design and implement a data structure forLeast Recently Used (LRU) cache. It should support the following operations:getandput.

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 inO(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

class LRUCache {
    private Node head;
    private Node tail;
    HashMap<Integer, Node> map;
    int capacity;
    public LRUCache(int capacity) {
        head = null;
        tail = null;
        this.capacity = capacity;
        map = new HashMap<Integer, Node>();
    }

    public int get(int key) {
        Node node = map.get(key);
        if (node == null) {
            return -1;
        }else{
            //don't forget to update the head pointer
            if (node != head) {
                if (node == tail) {
                    tail = tail.next;
                }else{
                    node.prev.next = node.next;
                    node.next.prev = node.prev;
                }
                head.next = node;
                node.prev = head;
                node.next = null;
                head = node;
            }

        }
        return node.val;
    }

    //                                     t         t head
    //              (1,1) <->     null<-(2,2)  <-> (3,3) <-> (1,3) -> null

     //       update (1,3)
    //


    public void put(int key, int value) {
        //two cases : if there exists the node for the key or not;
        Node node = map.get(key);
        if (node != null) {//if exists, update value, 
            node.val = value;
            if (node != head) {
                if (node == tail) {
                    tail = tail.next;  // delete the node for now and insert it into the head positon
                }else{
                // delete the node for now and insert it into the head positon
                    node.prev.next = node.next;
                    node.next.prev = node.prev;
                }
                //insert the node
                head.next = node;
                node.prev = head;
                node.next = null;
                head = node;
            }

        }else{
           node = new Node(key,value);
           if (capacity == 0) {
               Node temp = tail;
               tail = tail.next;
               map.remove(temp.key);

               capacity++;
           } //first remove and then add
           if (head == null && tail == null) {
               tail = node;
           }else{
               head.next = node;
               node.prev = head;
               node.next = null;
           }
            head = node;
            map.put(key, node); 
            capacity--;
        }
    }
    private class Node{
        int key;
        int val;
        Node prev;
        Node next;
        private Node(int key, int val) {
            this(key,val,null,null);
        }
        private Node(int key, int val, Node prev, Node next) {
            this.key = key;
            this.val = val;
            this.prev = prev;
            this.next = next;
        }
    }
}

/**
 * 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);
 */