Q200510-03-03 :LRU缓存机制

LRU缓存机制
运用你所掌握的数据结构，设计和实现一个 LRU (最近最少使用) 缓存机制。它应该支持以下操作： 获取数据 get 和 写入数据 put 。

获取数据 get(key) - 如果密钥 (key) 存在于缓存中，则获取密钥的值（总是正数），否则返回 -1。
写入数据 put(key, value) - 如果密钥已经存在，则变更其数据值；如果密钥不存在，则插入该组「密钥/数据值」。当缓存容量达到上限时，它应该在写入新数据之前删除最久未使用的数据值，从而为新的数据值留出空间。进阶:
你是否可以在 O(1) 时间复杂度内完成这两种操作？

示例:

LRUCache cache = new LRUCache( 2 /* 缓存容量 */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1); // 返回 1
cache.put(3, 3); // 该操作会使得密钥 2 作废
cache.get(2); // 返回 -1 (未找到)
cache.put(4, 4); // 该操作会使得密钥 1 作废
cache.get(1); // 返回 -1 (未找到)
cache.get(3); // 返回 3
cache.get(4); // 返回 4

代码:

package test3;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;

class Node {
    int key;
    int val;
    Node next;
    Node prev;

    public Node(int key, int val){
        this.key = key;
        this.val = val;
        next = null;
        prev = null;
    }
}

public class LruCache {
    private int capacity;
    private HashMap<Integer, Node> cacheMap;
    private Node head, tail;

    public LruCache(int capacity) {
        this.capacity = capacity;
        this.cacheMap = new LinkedHashMap<>();
        this.head = new Node(-1, -1);
        this.tail = new Node(-1, -1);
        head.next = tail;
        head.prev = tail;
        tail.next = head;
        tail.prev = head;
    }

    public int get(int key) {
        if(!cacheMap.containsKey(key)) {
            return -1;
        }
        Node node = cacheMap.get(key);
        moveToHead(node);
        return node.val;
    }

    public void put(int key, int value) {
        if (cacheMap.containsKey(key)) {
            cacheMap.get(key).val = value;
            moveToHead(cacheMap.get(key));
        } else {
            Node node = new Node(key, value);
            if (cacheMap.size() >= this.capacity) {
                Node rm = tail.prev;
                deleteNode(rm);
                cacheMap.remove(rm.key);
            }
            insertHead(node);
            cacheMap.put(key, node);
        }
    }

    private void moveToHead(Node node) {
        deleteNode(node);
        insertHead(node);
    }

    private void insertHead(Node node) {
        Node next = head.next;
        head.next = node;
        node.prev = head;
        node.next = next;
        next.prev = node;
    }

    private void deleteNode(Node node) {
        Node front = node.prev;
        Node end = node.next;
        front.next = end;
        end.prev = front;
    }

    public void printCache() {
        StringBuilder sb=new StringBuilder();
        sb.append("[");

        List<String> ls=new ArrayList<String>();
        for(int key:cacheMap.keySet()) {
            Node value=cacheMap.get(key);
            ls.add("("+key+","+value.val+")");
        }
        sb.append(String.join(",", ls));

        sb.append("]");
        System.out.println(sb.toString());
    }

    public static void main(String[] args) throws Exception{
        LruCache cache=new LruCache(2);
        cache.put(1,1);
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        cache.put(2,2);
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        System.out.print(cache.get(1));
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        cache.put(3, 3);
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        System.out.print(cache.get(2));
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        cache.put(4, 4);
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        System.out.print(cache.get(1));
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        System.out.print(cache.get(3));
        System.out.print(".....");
        cache.printCache();
        System.out.println();

        System.out.print(cache.get(4));
        System.out.print(".....");
        cache.printCache();
        System.out.println();
    }
}

输出:

.....[(1,1)]

.....[(1,1),(2,2)]

1.....[(1,1),(2,2)]

.....[(1,1),(3,3)]

-1.....[(1,1),(3,3)]

.....[(3,3),(4,4)]

-1.....[(3,3),(4,4)]

3.....[(3,3),(4,4)]

4.....[(3,3),(4,4)]

--2020年5月11日--