public IEnumerator <int> GetEnumerator()
 {
     if (root == null)
     {
         Enumerable.Empty <int>();
     }
     else
     {
         List <int> values = new List <int>();
         BinaryTreeUtilities.GetAllValues(root, values);
         foreach (var item in values)
         {
             yield return(item);
         }
     }
 }
 public void PrintLevelOrder()
 {
     BinaryTreeUtilities.PrintLevelOrder(root);
 }