思路：

使用链表结构模拟

代码：

let obj = {}; // 仿链表结构
let res = {}; // map结果
let cnt = 0; // 标记键值对个数
let pre = undefined; // 标记上一个
let head = undefined; // 标记链表头部节点
let tail = undefined; // 标记链表最后一个节点
function LRUCache(capacity) {
    function updateKeyVal(key) {
        if(head === obj[key]) { // 当前节点为头部
            head = obj[key].nextt || obj[key];
            head.pre = null;
            obj[key].pre = tail;
            tail = obj[key];
            obj[key].nextt = null;
            if(head.nextt === null) {
                head.nextt = tail;
            }
        } else if(tail !== obj[key]) { // 当前节点不能为尾部 尾部无需改变节点位置
            obj[key].pre.nextt = obj[key].nextt;
            obj[key].pre = tail;
            tail = obj[key];
            obj[key].nextt = null;
        }
    }
    this.set = function (key, val) {
        if(obj[key]){ // 更新
            updateKeyVal(key);
            obj[key] = {
                val: val,
            };
        } else { // 插入
            cnt++;
            if(cnt <= capacity) { // 有容量 直接在尾部插入新节点
                obj[key] = {
                    val: val,
                    key: key,
                    pre: null,
                    nextt: null,
                }
                if(pre === undefined) {
                    pre = obj[key];
                    head = obj[key];
                    tail = obj[key];
                } else {
                    pre.nextt = obj[key];
                    obj[key].pre = pre;
                    pre = obj[key];
                    tail = obj[key];
                }
            } else { // 容量已满 删除头部节点 并在尾部插入新节点
                cnt--;
                delete obj[head.key];
                delete res[head.key];
                head = head.nextt;
                head.pre = null;
                obj[key] = {
                    val: val,
                    key: key,
                    pre: tail,
                    nextt: null,
                }
                tail = obj[key];
                if(head.nextt === null) {
                    head.nextt = tail;
                }
            }
        }
        res[key] = val;
        // console.log(res, "====set=====");
        let result = head;
        let str = "{" + result.key + "=" +result.val;
        while (result.nextt) {
            result = result.nextt;
            str += ", " + result.key + "=" +result.val;
        }
        str += "}";
        console.log(str);
    }
    this.get = function (key) {
        if (obj[key]) {
            updateKeyVal(key);
            console.log(obj[key].val);
            return obj[key].val;
        } else {
            console.log(-1);
            return -1;
        }
    }
}
const cache = new LRUCache(2);
cache.set(1, 1); // 缓存是 {1=1}
cache.set(2, 2); // 缓存是 {1=1, 2=2}
cache.get(1);    // 返回 1
cache.set(3, 3); // 该操作会使得关键字 2 作废，缓存是 {1=1, 3=3}
cache.get(2);    // 返回 -1 (未找到)
cache.set(4, 4); // 该操作会使得关键字 1 作废，缓存是 {4=4, 3=3}
cache.get(1);    // 返回 -1 (未找到)
cache.get(3);    // 返回 3
cache.get(4);    // 返回 4