binaryTreeFlip

// interface TreeNode {
//   val: number;
//   left: TreeNode | null;
//   right: TreeNode | null;
// }
/**
 * @param {TreeNode | null} root
 * @return {TreeNode | null}
 */
// Time: O(n), Space: O(h) where h is the height of the tree
export default function binaryTreeFlip(root) {
  if (root === null) {
    return null;
  }

  // Swap left and right children

  [root.left, root.right] = [root.right, root.left];
  // Recursively invert subtrees
  binaryTreeFlip(root.left);
  binaryTreeFlip(root.right);

  return root;
}

// Input: root = [2,1,3]
// Output: [2,3,1]
// Explanation: The left and right children are swapped.