   public static void infix(BinaryTreeNode t)
   {
      if (t != null) 
      {
         System.out.print("(");     
         infix(t.leftChild);              // output left operand
         System.out.print(t.element);   // output operator
         infix(t.rightChild);             // output right operand
         System.out.print(")");
      }
   }
