public static class HtmlHelperExtensions
    {
        public static string CatTree(this HtmlHelper html, IEnumerable<Category> cats)
        {
            string htmlOutput = string.Empty;

            if (cats.Count() > 0)
            {
                htmlOutput += "<ul>";
                foreach (Category cat in cats)
                {
                    htmlOutput += "<li>";
                    htmlOutput += cat.Name;
                    htmlOutput += html.CatTree(cat.Categories);
                    htmlOutput += "</li>";
                }
                htmlOutput += "</ul>";
            }

            return htmlOutput;
        }
    }