LeetCode0146.java

/* LRU Cache
 * Input:
 * ["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
 * [[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
 * Output:
 * [null, null, null, 1, null, -1, null, -1, 3, 4]
 * */

import java.util.LinkedHashMap;

public class LeetCode0146 {
    public static void main(String args[]) {
        LRUCache LRUcache = new LRUCache(2);
        LRUcache.put(1, 1);
        LRUcache.put(2, 2);
        System.out.println(LRUcache.get(1));
        LRUcache.put(3, 3);
        System.out.println(LRUcache.get(2));
        LRUcache.put(4, 4);
        System.out.println(LRUcache.get(1));
        System.out.println(LRUcache.get(3));
        System.out.println(LRUcache.get(4));
    }
}

class LRUCache {
    LinkedHashMap<Integer, Integer> cache = new LinkedHashMap<>();
    int cap;

    public LRUCache(int capacity) {
        this.cap = capacity;
    }

    public int get(int key) {
        if (!cache.containsKey(key))
            return -1;
        move(key);
        return cache.get(key);
    }

    public void put(int key, int value) {
        if (cache.containsKey(key)){
            cache.put(key, value);
            move(key);
            return;
        }
        if (cache.size() >= cap){
            //System.out.println(cache.keySet());
            int oldest = cache.keySet().iterator().next();
            cache.remove(oldest);
        }
        cache.put(key, value);
    }

    public void move(int key){
        int val = cache.get(key);
        cache.remove(key);
        cache.put(key, val);
    }

    /* 不用LinkedHashMap，结合HashMap和双向列表（此方法更快）

    class DLinkedNode {
        int key;
        int value;
        DLinkedNode prev;
        DLinkedNode next;
        public DLinkedNode() {}
        public DLinkedNode(int _key, int _value) {key = _key; value = _value;}
    }

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

    public LRUCache(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        // 使用伪头部和伪尾部节点
        head = new DLinkedNode();
        tail = new DLinkedNode();
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            return -1;
        }
        // 如果 key 存在，先通过哈希表定位，再移到头部
        moveToHead(node);
        return node.value;
    }

    public void put(int key, int value) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            // 如果 key 不存在，创建一个新的节点
            DLinkedNode newNode = new DLinkedNode(key, value);
            // 添加进哈希表
            cache.put(key, newNode);
            // 添加至双向链表的头部
            addToHead(newNode);
            ++size;
            if (size > capacity) {
                // 如果超出容量，删除双向链表的尾部节点
                DLinkedNode tail = removeTail();
                // 删除哈希表中对应的项
                cache.remove(tail.key);
                --size;
            }
        }
        else {
            // 如果 key 存在，先通过哈希表定位，再修改 value，并移到头部
            node.value = value;
            moveToHead(node);
        }
    }

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

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

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

    private DLinkedNode removeTail() {
        DLinkedNode res = tail.prev;
        removeNode(res);
        return res;
    }
    */
}