public string GetInOrder()
        {
            var sb = new StringBuilder();

            if (LeftNode != null)
            {
                sb.Append(LeftNode.GetInOrder());
            }
            sb.Append(Element);
            sb.Append(",");
            if (RightNode != null)
            {
                sb.Append(RightNode.GetInOrder());
            }
            return(sb.ToString());
        }