Algs4-1.3链表实现不定容泛型Queue不支持迭代

 public class Queue<Item>
{
    private int N;
    private Node first;
    private Node last;
   
    private class Node
    {
        Item item;
        Node next;
    }
   
    public boolean isEmpty()
    {return N==0;}
   
    public int size()
    {return N;}
   
    public void enqueue(Item item)
    {
        Node oldlast=last;
        last=new Node();
        last.item=item;
        last.next=null;
        if(isEmpty())
            first=last;
        else
            oldlast.next=last;
        N++;
    }
   
    public Item dequeue()
    {
        Item item=first.item;
        first=first.next;
        if(isEmpty()) last=null;
        N--;
        return item;
   }
   
    public static void main(String[] args)
    {
        Queue<String> q=new Queue<String>();
        while(!StdIn.isEmpty())
        {
            String item=StdIn.readString();
            if(!item.equals("-"))
                q.enqueue(item);
            else if(!q.isEmpty())
                StdOut.print(q.dequeue()+" ");
        }//end while
        StdOut.println("("+q.size()+" left on Queue)");
    }//end main
}//end class