用对象模拟一个双端队列

 class Deque {
    constructor() {
      this.items={};
      this.count=0;
      this.lowestCount=0;
    }
    //一次增加一个元素
    addBack(element){
      this.items[this.count]=element;
      this.count++;
    }
    addFront(element){
      if(this.isEmpty()){
        this.addBack(element);
      }
      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.items[0]=element;
      }
    }
    isEmpty(){
      return this.count-this.lowestCount===0;
    }
    size(){
      return this.count-this.lowestCount;
    }
    removeFront(){
      if(this.isEmpty()){
        return undefined
      }
      let result=this.items[this.lowestCount];
      delete this.items[this.lowestCount];
      this.lowestCount++;
      return result;
    }
    removeBack(){
      if(this.isEmpty()){
        return undefined
      }
      this.count--;
      let result=this.items[this.count]
      delete this.items[this.count]
      return result;
    }
    peekFront() {
      if(this.isEmpty()){
        return undefined
      }
      return this.items[this.lowestCount];
    }
    peekBack() {
      if(this.isEmpty()){
        return undefined
      }
      return this.items[this.count-1];
    }
    clear() {
      this.items={};
      this.count=0;
      this.lowestCount=0;
    }
    toString() {
    if(this.isEmpty()){
      return ''
      }
    let ObjString=`${this.lowestCount}`
    for(let i=this.lowestCount+1;i<this.count;i++){
       ObjString=`${ObjString},${this.items[i]}`
    }
    return ObjString;
    }
  }
  let deque=new Deque();
  //deque.addBack(1)
 // deque.addBack(12)
  //deque.addBack(4)
 // deque.addBack(5)
 // deque.addBack(7)
 
 // deque. removeBack()

  deque.addFront(3)