I Don't Want to Hire You If You Can't Reverse a Binary Tree (2016)(thecodebarbarian.com)

  bool is_mirror(left_tree,right_tree) {

    if( !left_tree && !right_tree)
      return true;

    if( !left_tree || !right_tree)
      return false;

    return is_mirror(left_tree->left, right_tree->right)
      && is_mirror(left_tree->right, right_tree->left);

    }

  bool is_symmetric(tree) {
    !tree || is_mirror(tree-left,tree->right);
  }