0226-invert-binary-tree.dart

class Solution {
  TreeNode? invertTree(TreeNode? root) {
    if (root == null) {
      return null;
    }

    var tmp = root.left;
    root.left = root.right;
    root.right = tmp;

    invertTree(root.left);
    invertTree(root.right);

    return root;
  }
}