class Deque {
  constructor() {
    this.list = {}
    this.count = 0
    this.lowestrCount = 0
  }
  //在雙端隊(duì)列前端添加
  addFront(el){
    if(this.isEmpty()){
      this.addBack(el)
    } else if (this.lowestrCount>0){
      this.lowestrCount--
      this.list[this.lowestrCount] = element
    }else {
      for (let i =this.count;i >0;i--){
        this.list[i]=this.list[i-1]
      }
      this.count++
      this.lowestrCount=0
      this.list[0] = el
    }
  }
  //在雙端隊(duì)列后端添加
  addBack(el){
    this.list[this.count] = element
    this.count++
  }
  //刪除雙端隊(duì)列前端移除第一個(gè)元素
  removeFront (){
    if (this.isEmpty()) {
      return undefined
    }
    const result = this.list[this.lowestCount]
    delete this.list[this.lowestCount]
    this.lowestCount++
    return result
  }
  //刪除雙端隊(duì)列后端移除最后一個(gè)元素
  removeBack(){
    if (this.isEmpty()) {
      return undefined
    }
    const result = this.list[this.count]
    delete this.list[this.count]
    this.count--
    return result
  }
  //返回雙端隊(duì)列前端第一個(gè)元素
  peekFront(){
    if (this.isEmpty()) {
      return undefined
    }
    return this.list[this.lowestCount]
  }
  //返回雙端隊(duì)列后端第一個(gè)元素
  peekBack () {
    if (this.isEmpty()) {
      return undefined
    }
    return this.list[this.count]
  }
  //判斷雙端隊(duì)列是不是為空
  isEmpty() {
    return this.count-this.lowestrCount===0
  }
  //清空隊(duì)列
  clear () {
    this.list = {}
    this.count = 0
    this.lowestCount=0
  }
  //字符串方法
  toString () {
    if (this.isEmpty()) {
      return ''
    }
    let str = `${this.list[this.lowestCount]}`
    for (let i = this.lowestCount + 1; i < this.count; i++){
      str+=`,${this.list[i]}`
    }
    return str
  }
}