LRU 算法

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

/**
 * @since 2020/9/26 9:20
 */
public class LRUCache<K, V> {

    static class Node<K, V> {
        K key;
        V value;
        Node<K, V> pre;
        Node<K, V> next;

        public Node() {
        }

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

    private int size;
    private int capacity;
    private Node<K, V> head;
    private Node<K, V> tail;
    private final Map<K, Node<K, V>> cache = new HashMap<>();

    public LRUCache() {
        this(10);
    }

    public LRUCache(int capacity) {
        if (capacity <= 0) {
            throw new IllegalArgumentException("capacity <= 0");
        }
        this.size = 0;
        this.capacity = capacity;
        this.head = new Node<>();
        this.tail = new Node<>();
        head.next = tail;
        tail.pre = head;
    }

    public V get(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }
        Node<K, V> node = cache.get(key);
        if (node == null) {
            return null;
        }
        moveToHead(node);
        return node.value;
    }

    public void put(K key, V value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }
        Node<K, V> node = cache.get(key);
        if (node == null) {
            Node<K, V> newNode = new Node<>(key, value);
            cache.put(key, newNode);
            addToHead(newNode);
            ++size;
            if (size > capacity) {
                Node<K, V> tail = removeTail();
                cache.remove(tail.key);
                --size;
            }
        } else {
            node.value = value;
            moveToHead(node);
        }
    }

    private void moveToHead(Node<K, V> node) {
        removeNode(node);
        addToHead(node);
    }

    private void addToHead(Node<K, V> node) {
        node.pre = head;
        node.next = head.next;
        head.next.pre = node;
        head.next = node;
    }

    private void removeNode(Node<K, V> node) {
        node.pre.next = node.next;
        node.next.pre = node.pre;
    }

    private Node<K, V> removeTail() {
        Node<K, V> node = tail.pre;
        removeNode(node);
        return node;
    }
}