JavaScript) 큐 Queue

class Node{
  constructor(item){
    this.item = item;
    this.next = null;
  }
}

class Queue{
  constructor(){
    this.head = null;
    this.tail = null;
    this.length = 0;
  }

  push(item){
    const node = new Node(item)
    if(this.head==null){
      this.head= node;
    }else{
      this.tail.next = node;
    }

    this.tail = node;
    this.length +=1;
  }

  pop(){
    const popItem = this.head;
    this.head = this.head.next;
    this.length -=1;
    return popItem.item;
  }

  size(){
    return this.length;
  }

  empty(){
    if(this.length==0){
      return 1;
    }else{
      return 0;
    }
  }

  front(){
    if(this.empty()==1) return -1;
    return this.head.item; 
  }

  back(){
    if(this.empty()==1) return -1;
    return this.tail.item; 
  }
}