StringBuilder sb = new StringBuilder();
            string Table = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

            int[] p = new int[26];//字符索引
            int Length = 3 - 1;//字符串的长度
            int i = Length;
            p[i] = -1;
            //初始化sb变量
            int a =0;
            while (a++ <= Length) sb.Append("A");
            //遍历解空间
            while (i > -1)
            {
                if (p[i] == 25)
                {
                    sb.Remove(i, 1);
                    i--;
                    continue;
                }
                else
                {
                    p[i] = p[i] + 1;
                    sb[i] = Table[p[i]]; 
                    if (i < Length)
                    {
                        i++;
                        p[i] = -1;
                        sb.Append(" ");
                        continue;
                    }
                }
                Console.WriteLine ( sb.ToString() );
            }