计算a,b,c的排列组合

        public void SortAll(List<string> list,int start,int end)
        {
            if (start==end)
            {
                foreach (var item in list)
                {
                    Console.Write(item);
                }
                Console.WriteLine();
            }
            else
            {
                for (int i = start; i < end; i++)
                {
                    //交换变量
                    var temp = list[start];
                    list[start] = list[i];
                    list[i] = temp;
                    //递归排序
                    SortAll(list,start+1,end);
                    //还原变量
                    temp = list[start];
                    list[start] = list[i];
                    list[i] = temp;
                }
            }
        }