LRU算法详解

LRU作为redis淘汰机制一种，是淘汰最近最久未使用的算法，底层用的hashmap+双向链表

手写LRU:

import java.io.*;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Scanner;

public class LRU<K, V> implements Iterable<K> {
    private Node head;
    private Node tail;
    private HashMap<K, Node> map;
    private int maxSize;
    private class Node {
        Node pre;
        Node next;
        K k;
        V v;
        public Node(K k, V v) {
            this.k = k;
            this.v = v;
        }
    }
    public LRU(int maxSize) {
        this.maxSize = maxSize;
        this.map = new HashMap<>(maxSize * 4 / 3);
        head = new Node(null, null);
        tail = new Node(null, null);
        head.next = tail;
        tail.pre = head;
    }
    public V get(K key) {
        if (!map.containsKey(key)) {
            return null;
        }
        Node node = map.get(key);
        unlink(node);
        appendHead(node);
        return node.v;
    }
    public void put(K key, V value) {
        if (map.containsKey(key)) {
            Node node = map.get(key);
            unlink(node);
        }
        Node node = new Node(key, value);
        map.put(key, node);
        appendHead(node);
        if (map.size() > maxSize) {
            Node toRemove = removeTail();
            map.remove(toRemove.k);
        }
    }
    private void unlink(Node node) {
        Node pre = node.pre;
        Node next = node.next;
        pre.next = next;
        next.pre = pre;
        node.pre = null;
        node.next = null;
    }
    private void appendHead(Node node) {
        Node next = head.next;
        node.next = next;
        next.pre = node;
        node.pre = head;
        head.next = node;
    }
    private Node removeTail() {
        Node node = tail.pre;
        Node pre = node.pre;
        tail.pre = pre;
        pre.next = tail;
        node.pre = null;
        node.next = null;
        return node;
    }
    @Override
    public Iterator<K> iterator() {
        return new Iterator<K>() {
            private Node cur = head.next;
            @Override
            public boolean hasNext() {
                return cur != tail;
            }
            @Override
            public K next() {
                Node node = cur;
                cur = cur.next;
                return node.k;
            }
        };
    }
}