双端队列

/**
 * 双端队列
*/
class Deque {
  constructor() {
    // 队列当前索引
    this.count = 0
    // 队头索引
    this.lowestCount = 0
    // 存储队列
    this.items = {}
  }
  /**
   * 添加到队头
  */
  addFront(element) {
    if (this.isEmpty()) {
      this.addBack(element)
    } else if (this.lowestCount > 0) {
      this.lowestCount--
      this.items[this.lowestCount] = element
    } else {
      // 将所有元素向后移动一位
      for (let i = this.count; i > 0; i--) {
        this.items[i] = this.items[i - 1]
      }
      this.count++
      this.lowestCount = 0
      this.items[0] = element
    }
  }
  /**
   * 添加到队尾
  */
  addBack(element) {
    this.items[this.count] = element
    this.count++
  }
  /**
   * 移除队头
  */
  removeFront() {
    if (this.isEmpty()) {
      return undefined
    } else {
      const res = this.items[this.lowestCount]
      delete this.items[this.lowestCount]
      this.lowestCount++
      return res
    }
  }
  /**
   * 移除队尾
  */
  removeBack() {
    if (this.isEmpty()) {
      return undefined
    } else {
      this.count--
      const res = this.items[this.count]
      delete this.items[this.count]
      return res
    }
  }
  /**
   * 返回队头
  */
  peekFront() {
    if (this.isEmpty()) {
      return undefined
    } else {
      return this.items[this.lowestCount]
    }
  }
  /**
   * 添加到队尾
  */
  peekBack() {
    if (this.isEmpty()) {
      return undefined
    } else {
      return this.items[this.count - 1]
    }
  }
  /**
   * 查看队列是否为空
  */
  isEmpty() {
    return this.count - this.lowestCount === 0
  }
  /**
   * 查看队列长度
  */
  size() {
    return this.count - this.lowestCount
  }
  /**
   * 清空队列
  */
  clear() {
    this.count = 0
    this.lowestCount = 0
    this.items = {}
  }
}