队列

• 队列是遵循先进先出(FIFO，也称为先来先服务)原则的一组有序的项。队列在尾部添加新元素，并从顶部移除元素。最新添加的元素必须排在队列的末尾。

class Queue {
  constructor() {
    this.count = 0;
    this.lowestCount = 0;
    this.items = {};
  }
  enqueue(element) {
    //向队列尾部添加一个或者多个元素
    this.items[this.count] = element;
    this.count++;
  }
  dequeue() {
    //移除队列的第一项并返回被移除的元素
    if (this.isEmpty()) {
      return undefined;
    }
    const res = this.items[this.lowestCount];
    delete this.items[this.lowestCount];
    this.lowestCount++;
    return res;
  }
  peek() {
    //返回队列中的第一个元素
    if (this.isEmpty()) {
      return undefined;
    }
    return this.items[this.lowestCount];
  }
  isEmpty() {
    return this.count - this.lowestCount === 0;
  }
  size() {
    return this.count - this.lowestCount;
  }
  clear() {
    this.items = {};
    this.count = 0;
    this.lowestCount = 0;
  }
  toString() {
    if (this.isEmpty()) {
      return '';
    }
    let objString = `${this.items[this.lowestCount]}`;
    for (let i = this.lowestCount + 1; i < this.count; i++) {
      objString = `${objString},${this.items[i]}`;
    }
    return objString;
  }
}

双端队列

• 双端队列(deque，或称 double-endedqueue)是一种允许我们同时从前端和后端添加和移除元素的特殊队列。
• 在计算机科学中，双端队列的一个常见应用是存储一系列的撤销操作。每当用户在软件中进行了一个操作，该操作会被存在一个双端队列中(就像在一个栈里)。当用户点击撤销按钮时，该操作会被从双端队列中弹出，表示它被从后面移除了。在进行了预先定义的一定数量的操作后，最先进行的操作会被从双端队列的前端移除。由于双端队列同时遵守了先进先出和后进先出原则，可以说它是把==队列和栈相结合的一种数据结构==。

class Deque {
  constructor() {
    this.count = 0;
    this.lowestCount = 0;
    this.items = {};
  }
  isEmpty() {
    return this.count - this.lowestCount === 0;
  }
  size() {
    return this.count - this.lowestCount;
  }
  clear() {
    this.items = {};
    this.count = 0;
    this.lowestCount = 0;
  }
  toString() {
    if (this.isEmpty()) {
      return '';
    }
    let objString = `${this.items[this.lowestCount]}`;
    for (let i = this.lowestCount + 1; i < this.count; i++) {
      objString = `${objString},${this.items[i]}`;
    }
    return objString;
  }
  addFront(element) {
    if (this.isEmpty()) {
      this.addBack();
    } else if (this.lowestCount > 0) {
      this.lowestCount--;
      this.items[this.lowestCount] = element;
    } else {
      for (let i = this.count; i > 0; i--) {
        this.items[i] = this.items[i - 1];
      }
      this.count++;
      this.lowestCount = 0;
      this.items[0] = element;
    }
  }
  addBack(element) {
    this.items[this.count] = element
    this.count++;
  }
  removeFront() {
    if (this.isEmpty()) return undefined;
    const resFront = this.items[this.lowestCount];
    delete this.items[this.lowestCount];
    this.lowestCount++;
    return resFront;
  }
  removeBack() {
    if (this.isEmpty()) return undefined;
    this.count--;
    const resBack = this.items[this.count];
    delete this.items[this.lcount];
    return resBack;
  }
  peekFront() {
    if (this.isEmpty()) return undefined;
    return this.items[this.lowestCount];
  }
  peekBack() {
    if (this.isEmpty()) return undefined;
    return this.items[this.count - 1];
  }
}