solution.js

// DoubleLinkedList and Map
const dll = function (freq, ele, prev, next) {
  this.freq = freq;
  this.next = next ? next : null;
  this.prev = prev ? prev : null;
  this.ele = new Set();
  if (ele) {
    this.ele.add(ele);
  }
};
var AllOne = function () {
  this.nodeMap = new Map();
  this.head = null;
  this.tail = null;
};

/**
 * @param {string} key
 * @return {void}
 */
AllOne.prototype.addtodll = function (key, node) {
  let temp;
  if (!node) {
    if (!this.head) {
      temp = new dll(1, key);
      this.head = temp;
      this.tail = temp;
    } else {
      if (this.head.freq === 1) {
        temp = this.head;
        this.head.ele.add(key);
      } else {
        temp = new dll(1, key, null, this.head);
        this.head.prev = temp;
        this.head = temp;
      }
    }
  } else {
    let c_node = node;
    let n_node = node.next;
    let n_freq = node.freq + 1;
    if (n_node === null) {
      temp = new dll(n_freq, key, c_node, null);
      this.tail = temp;
      c_node.next = temp;
    } else if (n_node.freq === n_freq) {
      temp = n_node;
      temp.ele.add(key);
    } else {
      temp = new dll(n_freq, key, c_node, c_node.next);
      c_node.next = temp;
      temp.next.prev = temp;
    }

    c_node.ele.delete(key);
    if (c_node.ele.size === 0) {
      if (c_node.prev === null) {
        this.head = c_node.next;
        c_node.next.prev = null;
      } else {
        c_node.prev.next = temp;
        temp.prev = c_node.prev;
      }
    }
  }
  return temp;
};
AllOne.prototype.inc = function (key) {
  let c_node = this.addtodll(key, this.nodeMap.get(key));
  this.nodeMap.set(key, c_node);
};

/**
 * @param {string} key
 * @return {void}
 */
AllOne.prototype.removefromdll = function (key, node) {
  let c_freq = node.freq;
  let r_freq = c_freq - 1;
  let temp;
  if (c_freq === 1) {
    node.ele.delete(key);
    if (node.ele.size === 0) {
      if (this.head === this.tail) {
        this.head = null;
        this.tail = null;
      } else {
        this.head = node.next;
        node.next.prev = null;
      }
    }
    return null;
  } else {
    if (node.prev === null) {
      temp = new dll(r_freq, key, null, node);
      this.head = temp;
      node.prev = temp;
    } else if (node.prev.freq === r_freq) {
      temp = node.prev;
      temp.ele.add(key);
    } else {
      temp = new dll(r_freq, key, node.prev, node);
      node.prev.next = temp;
      node.prev = temp;
    }

    node.ele.delete(key);
    if (node.ele.size === 0) {
      if (node.next === null) {
        this.tail = node.prev;
        temp.next = null;
      } else {
        node.next.prev = temp;
        temp.next = node.next;
      }
    }
  }
  return temp;
};
AllOne.prototype.dec = function (key) {
  let c_node = this.removefromdll(key, this.nodeMap.get(key));
  if (c_node) {
    this.nodeMap.set(key, c_node);
  } else {
    this.nodeMap.delete(key);
  }
};

/**
 * @return {string}
 */
AllOne.prototype.getMaxKey = function () {
  if (this.tail) {
    const [f] = this.tail.ele;
    return f;
  }
  return "";
};

/**
 * @return {string}
 */
AllOne.prototype.getMinKey = function () {
  if (this.head) {
    const [f] = this.head.ele;
    return f;
  }
  return "";
};

/**
 * Your AllOne object will be instantiated and called as such:
 * var obj = new AllOne()
 * obj.inc(key)
 * obj.dec(key)
 * var param_3 = obj.getMaxKey()
 * var param_4 = obj.getMinKey()
 */