class Queue[T] {
  private[this] class Node[T](val value:T) {
    var next:Option[Node[T]]=None
    def append(n:Node[T])=next=Some(n)
  }
  private[this] var head:Option[Node[T]]=None
  private[this] var tail:Option[Node[T]]=None

  def isEmpty=head.isEmpty

  def enqueue(item:T)={
    val n=new Node(item)
    if(isEmpty) head=Some(n) else tail.get.append(n)
    tail=Some(n)
  }

  def dequeue:T=head match {
    case Some(item) => head=item.next; item.value
    case None => throw new java.util.NoSuchElementException()
  }

  def front:T=head match {
    case Some(item) => item.value
    case None => throw new java.util.NoSuchElementException()
  }

  def iterator: Iterator[T]=new Iterator[T]{
    private[this] var it=head;
    override def hasNext=it.isDefined
    override def next:T={val n=it.get; it=n.next; n.value}
  }

  override def toString()=iterator.mkString("Queue(", ", ", ")")
}