The root is still the root, but everything else is just flipped: https://leetcode.com/problems/invert-binary-tree/

It is also extremely simple:

  TreeNode* invertTree(TreeNode* root) {
    if (root) {
      invertTree(root->left);
      invertTree(root->right);
      std::swap(root->left, root->right);
    }
    return root;
  }