List<int> fun(ref List<int> intcount, int a)
        {
            if (1 == a)
                return intcount;
            else
            {
                List<int> ii = new List<int>(); ii.Add(a - 1);
                intcount.AddRange(ii);
                return fun(ref intcount, a - 1);
            }
        }

 List<int> intcounts = new List<int>();
fun(ref intcounts, 10);
//返回的intcounts的结果是 9,8,7,6,5,4,3,2,1