题目

思路

1. O(1) 查询 (哈希表)
2. 记录最近使用顺序 (链表)
3. 需要组合哈希表 + 双向链表

Java 解法

class LRUCache {

    private static class Node {
        int key, val;
        Node prev, next;
        Node(int k, int v) {
            key = k;
            val = v;
        }
    }

    private final int capacity;
    private final Map<Integer, Node> map = new HashMap<>();
    private final Node head = new Node(0, 0);
    private final Node tail = new Node(0, 0);

    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        Node node = map.get(key);
        if (node == null) {
            return -1;
        }
        moveToHead(node);
        return node.val;
    }
    
    public void put(int key, int value) {
        Node node = map.get(key);
        if (node != null) {
            node.val = value;
            moveToHead(node);
            return;
        }

        Node n = new Node(key, value);
        map.put(key, n);
        addAfterHead(n);
        if (map.size() > capacity) {
            Node lru = removeTail();
            map.remove(lru.key);
        }
    }

    private void moveToHead(Node node) {
        remove(node);
        addAfterHead(node);
    }

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

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

    private Node removeTail() {
        Node node = tail.prev;
        remove(node);
        return node;
    }
}