input: Write a function to drop all leaf nodes from a given Huffman tree. The function should take the root of the tree as input and return the root of the modified tree without leaf nodes.
  assert dropLeafNodes((1, (2, (4, None, None), (5, None, None)), (3, (6, None, None), (7, None, None))))==(1, (2, None, None), (3, None, None))
  assert dropLeafNodes((1, None, None))==None
  assert dropLeafNodes((1, (2, None, None), (3, None, None)))==(1, None, None)
  assert dropLeafNodes(None)==None
  assert dropLeafNodes((1, (2, (3, None, None), None), (4, None, None)))==(1, (2, None, None), None)
output: def dropLeafNodes(root):
      if root is None:
          return None
      if root[1] is None and root[2] is None:
          return None
      root = (root[0], dropLeafNodes(root[1]), dropLeafNodes(root[2]))
      if root[1] is None and root[2] is None:
          return None
      return root