.net中string无重复数字的实现方法

//string strtext = "1334677999";实现无重复
        public static string qu(string lab)
        {
            string strresult = string.empty;
            for (int intidx = 0; intidx < lab.length; intidx++)
            {
                if (strresult.indexof(lab.substring(intidx, 1)) < 0)
                {
                    strresult += lab.substring(intidx, 1);
                }
            }
            return strresult;
        }