【LeetCode】LRU Cache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(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.

import java.util.HashMap;
import java.util.LinkedList;



class ListNode{
	int key, value;
	ListNode next, prev;
	public ListNode()
	{
		this.key = this.value = 0;
		next = prev = null;
	}
	public ListNode(int key, int val)
	{
		this.key = key;
		this.value = val;
		next = prev = null;
	}
}

public class LRUCache {
	private ListNode head,tail;
	private HashMap<Integer, ListNode> hashmap;
	private int size, capacity;

	public static void main(String[] args) {
		// TODO Auto-generated method stub

	}
	
	public LRUCache(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        head = new ListNode();
        tail = new ListNode();
        head.next = tail;
        tail.prev = head;
        hashmap = new HashMap<Integer, ListNode>();
    }
    
    public int get(int key) {
        if (hashmap.containsKey(key)) {
			ListNode cur = hashmap.get(key);
			//detach the node in the list
			cur.prev.next = cur.next;
			cur.next.prev = cur.prev;
			//set the visting node be the firstNode.
			ListNode tmp = head.next;
			head.next = cur;
			cur.prev = head;
			cur.next = tmp;
			tmp.prev = cur;
			return cur.value;
		}
        else return -1;
    }
    
    public void set(int key, int value) {
        if (hashmap.containsKey(key)) {
			ListNode cur = hashmap.get(key);
			cur.value = value;
			//detach
			cur.prev.next = cur.next;
			cur.next.prev = cur.prev;
			//set the visting node be the firstNode.
			ListNode tmp = head.next;
			head.next = cur;
			cur.prev = head;
			cur.next = tmp;
			tmp.prev = cur;
		}
        else {
			if (size == capacity) {
				ListNode last = tail.prev;
				hashmap.remove(last.key);
				tail = last;
				last = null;
				size--;
			}
			ListNode cur = new ListNode(key, value);
			hashmap.put(key, cur);
			//set the new node be the firstNode.
			ListNode tmp = head.next;
			head.next = cur;
			cur.prev = head;
			cur.next = tmp;
			tmp.prev = cur;
			size++;
		}
    }

}