[LC] 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.

The cache is initialized with a positive capacity.

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

class LRUCache {
    private int capacity;
    private Map<Integer, Node> map;
    private Node head;
    private Node tail;
    // hash map makes O(1) to get and put
    // doubly LL makes removing easier
    public LRUCache(int capacity) {
        this.map = new HashMap<>();
        this.capacity = capacity;
        this.head = null;
        this.tail = null;
    }

    public int get(int key) {
        Node curNode = map.get(key);
        if (curNode == null) {
            return -1;
        }

        if (tail != curNode) {
            if (curNode == head) {
                head = head.next;
            } else {
                curNode.prev.next = curNode.next;
                curNode.next.prev = curNode.prev;
            }
            tail.next = curNode;
            curNode.prev = tail;
            tail = curNode;
        }
        return curNode.value;
    }

    public void put(int key, int value) {
        Node curNode = map.get(key);
        if (curNode != null) {
　　　　　　　// update the current Node, for get(), similiar with this snippet
            curNode.value = value;
            if (tail != curNode) {
                if (curNode == head) {
                    head = head.next;
                } else {
                    curNode.prev.next = curNode.next;
                    curNode.next.prev = curNode.prev;
                }
                tail.next = curNode;
                curNode.prev = tail;
                tail = curNode;
            }
        } else {
            Node newNode = new Node(key, value);
            if (capacity == 0) {
                Node tmp = head;
                head = tmp.next;
                map.remove(tmp.key);
                capacity += 1;
            }
            if (head == null && tail == null) {
                head = newNode;
            } else {
                tail.next = newNode;
                newNode.prev = tail;
            }
            tail = newNode;
            capacity -= 1;
            map.put(key, newNode);
        }
    }
}

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

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

/**
 * 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);
 */