Leetcode 146 LRU

实现LRU缓存， 学会两种实现方式， 原始一点的用HashMap,可以了解更多实现细节， 还有就是LinkedHashMap(比较简单 就省略了)（ 可以先将代码放到Leetcode上跑通过后再来看代码）， 看了半天别人的代码然后跑过去通不过是件很沮丧的事情。

1. HashMap版本

package com.azhou.xiaoming;

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

public class LRUCache {
    int capacity;
    Map map = new HashMap<>();
    LRUNode head;
    LRUNode tail;
    public LRUCache(int capacity) {
        this.capacity = capacity;
    }

    public int get(int key) {
        LRUNode node = map.get(key);
        if(node != null) {
            removeAndInsert(node);
            return node.value;
        }
        return -1;
    }

    public void put(int key, int value) {
        if(head == null) {
            head = new LRUNode(key, value);
            tail = head;
            map.put(key, head);
        }

        LRUNode temp = map.get(key);
        if(temp != null) {
            temp.value = value;
            map.put(key, temp);
            removeAndInsert(temp);
        } else {

            if(map.size() >= capacity) {
                map.remove(tail.key);
                if(tail == head) {
                    head = new LRUNode(key, value);
                    tail = head;
                    map.put(key, head);
                    return ;
                } else {
                    tail = tail.pre;
                    tail.next = null;
                }
            }
            LRUNode node = new LRUNode(key, value);
            map.put(key, node);
            node.next = head;
            head.pre = node;
            node.pre = null;
            head = node;
        }
    }

    private void removeAndInsert(LRUNode node) {
        if(node == head) {
            return ;
        } else if(node == tail) {
            tail = node.pre;
            tail.next = null;
        } else {
            node.pre.next = node.next;
            node.next.pre = node.pre;
        }

        node.next = head;
        node.pre = null;
        head.pre = node;
        head = node;


    }

    public static void main(String[] args) {
        LRUCache cache = new LRUCache(1);
        cache.put(1,1);
        int i = cache.get(1);
        System.out.println("第一次：" + i);
        cache.put(2,2);
        int i1 = cache.get(2);
        System.out.println("第二次:" + i1);
        cache.put(3,3);
    }

}

class LRUNode {
    int key;
    int value;
    LRUNode next;
    LRUNode pre;

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