LRU Cache

题目原型：

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.

思路：

LRU是Least Recently Used 近期最少使用算法。在操作系统中，我们计算时，假如此时访问一个内存单元，那么我们把这个内存单元的标记加1，然后把其他的内存单元标记减1，当cache满时，我们替换标记值最小的单元。下面看个例子：

首先，1加进来，flag1=1；2加进来，flag2=1，flag1-1=0；2加进来，flag3=1，flag2-1=0，flag1-1=-1；此时4加进来，cache满了，我们替换最小的flag1，那么就是把4放到1的位置上去，以此类推；

代码如下：

Input:   2,[set(2,1),set(1,1),get(2),set(4,1),get(1),get(2)]

Expected:  [1,-1,1]

public class LRUCache
{
	private HashMap<Integer, Node> map = new HashMap<Integer, Node>(); 
	private int capacity;
	private Node head = new Node(0, 0);
	private Node tail = new Node(0, 0);
	
	
	public LRUCache(int capacity) 
	{
        this.capacity = capacity;
        head.next = tail;
        tail.pre = head;
    }
    //相当于访问，所以要重新调整链表的次序，使得刚才被访问的节点放在头结点后
    public int get(int key) 
    {
        if(!map.containsKey(key))
        	return -1;
        Node current = map.get(key);
        current.next.pre = current.pre;
        current.pre.next = current.next;
        add(current);//重新调整链表顺序
        return current.value;
    }
    //插入，如果cache中存在了，则调整链表次序，此时有可能value不能，则需要更新value值；否则，如果容量满了，则应删除最不常访问的节点，然后再添加。
    public void set(int key, int value) 
    {
    	//如果已经存在
    	if(map.containsKey(key))
    	{
    		Node current = map.get(key);
    		current.value = value;
    		current.next.pre = current.pre;
            current.pre.next = current.next;
            add(current);//重新调整链表顺序
    	}
    	else
    	{
    		if(map.size()==this.capacity)
    			delete();
    		Node current = new Node(key, value);
    		add(current);
    		map.put(key, current);
    	}
    }
    
    public void add(Node current)
    {
    	current.next = head.next;
    	head.next.pre = current;
    	head.next = current;
    	current.pre = head;
    }
    
    public void delete()
    {
    	Node temp = tail.pre;
    	tail.pre = temp.pre;
    	temp.pre.next = tail;
    	temp.next = null;
    	temp.pre = null;
    	map.remove(temp.key);
    }
}

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

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