input: def dropLeafNodes(head):
      """
      Implement a function that takes a Huffman tree and drop all its leaf nodes.
      
      >>> dropLeafNodes(None)
      None
      >>> dropLeafNodes(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3, TreeNode(6), TreeNode(7))))
      TreeNode(1)
      >>> dropLeafNodes(TreeNode(2, TreeNode(4), TreeNode(5)))
      None
      """
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)
      
      if head.left is None and head.right is None:
          return None
      
      return head