面试题 16.25. LRU缓存

注：这里采用的是单链表＋hashmap实现。效率没有双链表高。 双链表的插入删除都是O（1）

package 腾讯笔试;

import java.util.HashMap;
import java.util.Map;

/**
 * @author YJS
 * @date 2020/8/26 10:11
 */

public class LRUCache {
    class DLinkedNode {
        int key;
        int value;
        DLinkedNode prev;
        DLinkedNode next;
        public DLinkedNode() { }
        public DLinkedNode(int key,int value) {
            this.key = key;
            this.value = value;
        }
    }

    private Map<Integer, DLinkedNode> map = new HashMap<>();
    private int size;
    private int capacity;
    private DLinkedNode head, tail;

    public LRUCache(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        head = new DLinkedNode();
        tail = new DLinkedNode();
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        DLinkedNode node = map.get(key);
        if(node==null) return -1;
        moveToHead(node);
        return node.value;
    }

    public void put(int key, int value) {
        DLinkedNode node = map.get(key);
        if (node == null) {
            DLinkedNode newNode = new DLinkedNode(key, value);
            map.put(key, newNode);
            addToHead(newNode);
            ++size;
            if (size > capacity) {
                DLinkedNode remNode = removeTail();
                map.remove(remNode.key);
            }
        } else {
            node.value = value;
            moveToHead(node);
        }
    }

    private DLinkedNode removeTail() {
        DLinkedNode node = tail.prev;
        removeNode(node);
        return node;
    }

    private void moveToHead(DLinkedNode node) {
        removeNode(node);
        addToHead(node);
    }

    private void removeNode(DLinkedNode node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }

    private void addToHead(DLinkedNode node) {
        node.prev = head;
        node.next = head.next;
        head.next.prev = node;
        head.next = node;
    }
}