LRU缓存

LRU缓存（Least Recently Used，最近最少使用）是一种数据缓存机制，旨在解决计算机内存中数据的替换问题。当缓存空间不足时，LRU缓存会淘汰最近最久未被使用的数据，以确保缓存中始终存储着最新和最频繁使用的数据。

class LRUCache {
class Node{
    int key,val;
    Node pre,next;
    public Node(){}
    public Node(int key,int val){
        this.key = key;
        this.val = val;
    }
}
private ConcurrentHashMap<Integer,Node> cache = new ConcurrentHashMap<>();
private int size;
private int capacity;
private Node head = new Node();
private Node tail = new Node();
    public LRUCache(int capacity) {
        this.capacity = capacity;
        size = 0;
        head.next = tail;
        tail.pre = head;
    }
    
    public int get(int key) {
        Node node = cache.get(key);
        if(node == null)
            return -1;
        moveToHead(node);
        return node.val;
    }
    
    public void put(int key, int value) {
        Node node = cache.get(key);
        if(node != null){
            node.val = value;
            moveToHead(node);
        }else{
            Node newNode = new Node(key,value);
            cache.put(key,newNode);
            addToHead(newNode);
            ++size;
            if(size > capacity){
                Node removeNode = removeTail();
                cache.remove(removeNode.key);
                --size;
            }
        }
    }
    public void moveToHead(Node node){
        removeNode(node);
        addToHead(node);
    }
    public void removeNode(Node node){
        node.pre.next = node.next;
        node.next.pre = node.pre;
    }
    public void addToHead(Node node){
        node.pre = head;
        node.next  = head.next;
        head.next.pre = node;
        head.next = node;
    }
    public Node removeTail(){
        Node removeNode = tail.pre;
        removeNode(removeNode);
        return removeNode;
    }
}