// Adds newEntry to the nonempty subtree rooted at rootNode.
private T addEntry(BinaryNode<T> rootNode, T newEntry)
{
   assert rootNode != null;
   T result = null;
   int comparison = newEntry.compareTo(rootNode.getData());

   if (comparison == 0)
   {
      result = rootNode.getData();
      rootNode.setData(newEntry);
   }
   else if (comparison < 0)
   {
      if (rootNode.hasLeftChild())
         result = addEntry(rootNode.getLeftChild(), newEntry);
      else
         rootNode.setLeftChild(new BinaryNode<>(newEntry));
   }
   else
   {
      assert comparison > 0;

      if (rootNode.hasRightChild())
         result = addEntry(rootNode.getRightChild(), newEntry);
      else
         rootNode.setRightChild(new BinaryNode<>(newEntry));
   } // end if

   return result;
} // end addEntry
// Version 4.0