面试题：手写LRU

/**
 * @author WGR
 * @create 2021/1/12 -- 17:12
 */
public class LRUCacheDemo2<K,V> extends LinkedHashMap<K,V> {

    private int capacity;

    public LRUCacheDemo2(int capacity){
        super(capacity,0.75F,true);
        this.capacity = capacity;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return super.size() > capacity;
    }

    public static void main(String[] args) {
        LRUCacheDemo2 lruCacheDemo = new LRUCacheDemo2(3);
        lruCacheDemo.put(1,1);
        lruCacheDemo.put(2,2);
        lruCacheDemo.put(3,3);
        System.out.println(lruCacheDemo.keySet());
        lruCacheDemo.put(4,1);
        System.out.println(lruCacheDemo.keySet());
        lruCacheDemo.put(3,1);
        System.out.println(lruCacheDemo.keySet());
        lruCacheDemo.put(3,1);
        System.out.println(lruCacheDemo.keySet());
        lruCacheDemo.put(5,1);
        System.out.println(lruCacheDemo.keySet());
    }
}

/**
 * @author WGR
 * @create 2021/1/12 -- 15:54
 */
public class LRUCacheDemo {

    //构造一个Node节点，作为数据载体
    class Node<K,V>{
        K key;
        V value;
        Node<K,V> prev;
        Node<K,V> next;
        public Node(){
            this.prev = this.next = null;
        }

        public Node(K key,V value){
            this.key = key;
            this.value = value;
            this.prev = this.next = null;
        }
    }

    //构造一个双向队列，里面放Node
    class DoubleLinkedList<K,V>{
        Node<K,V> head;
        Node<K,V> tail;
        public DoubleLinkedList(){
           head = new Node<>();
           tail =  new Node<>();
           head.next = tail;
           tail.prev = head;
        }

        public void addHead(Node<K,V> node){
            node.next = head.next;
            node.prev = head;
            head.next.prev = node;
            head.next = node;
        }

        public void removeNode(Node<K,V> node){
            node.next.prev = node.prev;
            node.prev.next = node.next;
            node.prev = null;
            node.next = null;
        }

        public Node<K,V> getLast(){
            return tail.prev;
        }
    }
    public  int cacheSize;
    Map<Integer,Node<Integer,Integer>> map;
    DoubleLinkedList<Integer,Integer> doubleLinkedList;

    public LRUCacheDemo(int cacheSize){
        this.cacheSize = cacheSize;
        map = new HashMap<>();
        doubleLinkedList = new DoubleLinkedList<>();
    }

    public int get(int key){
        if(!map.containsKey(key)){
            return -1;
        }
        Node<Integer, Integer> node = map.get(key);
        doubleLinkedList.addHead(node);

        return node.value;

    }

    public void put(int key,int value){
        if(map.containsKey(key)){
            Node<Integer, Integer> node = map.get(key);
            node.value = value;
            map.put(key,node);
            doubleLinkedList.removeNode(node);
            doubleLinkedList.addHead(node);
        }else{
            if(map.size() == cacheSize){
                Node<Integer, Integer> lastNode = doubleLinkedList.getLast();
                map.remove(lastNode.key);
                doubleLinkedList.removeNode(lastNode);
            }
            Node<Integer, Integer> newNode = new Node<>(key, value);
            map.put(key,newNode);
            doubleLinkedList.addHead(newNode);
        }
    }

    public static void main(String[] args) {
        LRUCacheDemo lruCacheDemo = new LRUCacheDemo(3);
        lruCacheDemo.put(1,1);
        lruCacheDemo.put(2,2);
        lruCacheDemo.put(3,3);
        System.out.println(lruCacheDemo.map.keySet());
        lruCacheDemo.put(4,1);
        System.out.println(lruCacheDemo.map.keySet());
        lruCacheDemo.put(3,1);
        System.out.println(lruCacheDemo.map.keySet());
        lruCacheDemo.put(3,1);
        System.out.println(lruCacheDemo.map.keySet());
        lruCacheDemo.put(5,1);
        System.out.println(lruCacheDemo.map.keySet());
    }


}