input: def dropLeafNodes(head):
      """
      Given a Huffman tree, drop all its leaf nodes.
      
      >>> dropLeafNodes(None)
      None
      >>> dropLeafNodes(HuffmanNode(1, HuffmanNode(2, HuffmanNode(4), HuffmanNode(5)), HuffmanNode(3, HuffmanNode(6), HuffmanNode(7))))
      HuffmanNode(1, HuffmanNode(2), HuffmanNode(3))
      """
output:     if head is None:
          return None
      
      if head.left is None and head.right is None:
          return None
      
      head.left = dropLeafNodes(head.left)
      head.right = dropLeafNodes(head.right)
      
      return head