public Iterator<T> iteratorLevelOrder() 
   {
      ArrayUnorderedList<BinaryTreeNode<T>> nodes = new ArrayUnorderedList<BinaryTreeNode<T>>();
          
      ArrayUnorderedList<T> result = new ArrayUnorderedList<T>();
      
      BinaryTreeNode<T> current;

      nodes.addToRear (root);

      while (! nodes.isEmpty()) 
	 {
         current = (BinaryTreeNode<T>)nodes.removeFirst();

         if (current != null) 
	    {
            result.addToRear(current.getElement());
            nodes.addToRear (current.getLeft());
            nodes.addToRear (current.getRight());
         }//if
         else
            result.addToRear(null);
      }//while
      
      return result.iterator();
   }