226. Invert Binary Tree

  public TreeNode InvertTree(TreeNode root)
        {
            if (root == null)
            {
                return null;
            }

            var left = root.left;
            var right = root.right;
            if (left == null && right == null)
            {
                return root;
            }

            if (left != null)
            {
                root.right = InvertTree(left);
            }
            else
            {
                root.right = null;
            }

            if (right != null)
            {
                root.left = InvertTree(right);
            }
            else
            {
                root.left = null;
            }

            return root;
        }