input: Write a function to mirror a binary tree.
  assert mirrorBinaryTree(Node(1)) is None
  assert mirrorBinaryTree(None) is None
  assert mirrorBinaryTree(Node(1).left) is None
output: def mirrorBinaryTree(node):
      if node is None:
          return None
      node.left, node.right = node.right, node.left
      mirrorBinaryTree(node.left)
      mirrorBinaryTree(node.right)
      return node