private static void PostOrder(LeetCode590Node root)
        {
            if (root == null)
            {
                return;
            }

            foreach (var child in root.children)
            {
                PostOrder(child);
            }
            results.Add(root.val);
        }
 public static IList <int> PostOrderTraversal(LeetCode590Node root)
 {
     PostOrder(root);
     return(results);
 }