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

class Queue {
  constructor() {
    this.head = null;
    this.tail = null;
    this.size = 0;
  }
  
  enqueue(value) {
    let node= new Node(value)
    
    if(this.isEmpty()) {
      this.head = node
      this.tail = node
    } else {
      this.tail.next = node
      this.tail = node
    }
    
    this.size++
  }
  
  dequeue(){
    if(this.isEmpty()) return
    
    const removedNode = this.head
    
    if (this.size === 1) {
      this.head = null
      this.tail = null
      this.size-- 
      
      return removedNode.value
    }
    
    this.head = removedNode.next
    this.size--
    return removedNode.value
  }
  
  peek() {
    if(this.isEmpty()) return
    return this.head.value
  }
  
  isEmpty() {
    return this.size === 0
  }
  
  print() {
    if(this.isEmpty()) return
    
    let data = this.head
    
    while(data) {
      console.log(data.value)
      data = data.next
    }
  }
}

let queue = new Queue()
queue.enqueue(5)
queue.enqueue(10)
queue.enqueue(15)
queue.enqueue(20)
console.log(`POPed Value: ${queue.dequeue()}`)
console.log(`Head/peek Value: ${queue.peek()}`)
queue.print()