input: Write a function dropLeafNodes(head) that takes a Huffman tree and returns a new tree with all its leaf nodes removed. If the input tree is empty or fully consists of leaf nodes, return None.
  assert dropLeafNodes(None)==None, "Test case 1 failed"
  assert dropLeafNodes(Node(2, Node(4), Node(5)))==Node(2, None, None), "Test case 2 failed"
  assert dropLeafNodes(Node(1, Node(2, None, None), Node(3, Node(4), Node(5))))==Node(1, None, Node(3, None, None)), "Test case 3 failed"
  assert dropLeafNodes(Node(1, Node(2, Node(3), Node(4)), Node(5)))==Node(1, Node(2, None, None), None), "Test case 4 failed"
output: def dropLeafNodes(head):
      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