public String Multiply(string num1, string num2)

        {

            int num1Length = num1.Length;

            int num2Length = num2.Length;

 

            //tranfor the string to int array

            int[] inum1 = new int[num1Length];

            //先将字符串转化为整数数组(每个数字字符减去48即可)

            for (int i = 0; i < num1Length; i++)

            {

                inum1[i] = (int)num1[i] - 48;

            }

            int[] inum2 = new int[num2Length];

            for (int i = 0; i < num2Length; i++)

            {

                inum2[i] = (int)num2[i] - 48;

            }

            int[] result = new int[num1Length + num2Length];

            for (int i = num1Length - 1; i >= 0; i--)

            {

                for (int j = num2Length - 1; j >= 0; j--)

                {

                    result[i + j + 1] += inum1[i] * inum2[j];

                    result[i + j] += result[i + j + 1] / 10;

                    result[i + j + 1] %= 10;

                }

            }

            char[] temp = new char[num1Length + num2Length];

            for (int i = 0; i < (num1Length + num2Length); i++)

            {

                temp[i] = (char)(result[i] + 48);

            }

            String sResult = new String(temp);

            return sResult;

        }