LeetCode146 LRU Cache

题目：

       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

import java.util.HashMap;

public class LRUCache {

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

        public Node(){

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

    private Node head;
    private Node tail;
    private HashMap map;
    private int capacity;
    private int count;

    public LRUCache(int capacity) {
        map = new HashMap<>();
        this.capacity = capacity;
        this.count = 0;
        head = new Node();
        tail = new Node();

        head.pre = null;
        tail.next = null;

        head.next = tail;
        tail.pre = head;
    }

    public int get(int key) {
        Node node = map.get(key);
        if (node == null)
            return -1;
        deleteNode(node);
        addToHead(node);
        return node.value;
    }

    public void put(int key, int value) {
        Node node = map.get(key);
        if (node == null){
            Node newNode = new Node();
            newNode.key = key;
            newNode.value = value;
            addToHead(newNode);
            map.put(key, newNode);
            count++;
            if (count > capacity){
                Node tailNode = popTail();
                map.remove(tailNode.key);
                count--;
            }
        }else {
            node.value = value;
            deleteNode(node);
            addToHead(node);
        }
    }

    private void addToHead(Node node){
        node.pre = head;
        node.next = head.next;
        head.next.pre = node;
        head.next = node;
    }

    private void deleteNode(Node node){
        Node pre = node.pre;
        Node next = node.next;
        pre.next = next;
        next.pre = pre;
    }

    private Node popTail(){
        Node node = tail.pre;
        deleteNode(node);
        return node;
    }

    public void print(){
        Node node = head.next;
        while (node.next != null){
            System.out.print("(" + node.key +", " + node.value + ")" + "=>");
            node = node.next;
        }
        System.out.println("NULL");
    }

    public static void main(String[] args) {
        LRUCache cache = new LRUCache(2);
        cache.put(1, 1);
        cache.print();
        cache.put(2, 2);
        cache.print();
        System.out.println(cache.get(1));
        cache.print();
        cache.put(3, 3);
        cache.print();
        System.out.println(cache.get(2));
        cache.print();
        cache.put(4, 4);
        cache.print();
        System.out.println(cache.get(1));
        cache.print();
        System.out.println(cache.get(3));
        cache.print();
        System.out.println(cache.get(4));
        cache.print();
    }
}