删除字符串中重复的字符

需要用到ArrayList 和 StringBuilder

        static void Main(string[] args)
        {
            string str = "ABCDEFABCD";
            string test = DeleteCommon(str);
            Console.WriteLine(test);
        }

        private static string DeleteCommon(string str)
        {
            ArrayList arrayList = new ArrayList();
            foreach (char c in str)
            {
                if (!arrayList.Contains(c))
                {
                    arrayList.Add(c);
                }
            }

            StringBuilder strb = new StringBuilder();
            foreach (char c in arrayList)
            {
                strb.Append(c);
            }

            return strb.ToString();
        }