大树相乘算法

private string Multiplication(string num1, string num2)
    {
        var input1 = num1.Select(i => (int)(i - '0'));
        var input2 = num2.Select(i => (int)(i - '0'));

        //把num1依次和num2每位相乘，获取到一个m*n的矩阵
        var rectangel = input2.Select(i => input1.Select(j => j * i).ToArray()).ToArray();

        //将矩阵错位相加，得到输出数组
        var result = new int[num1.Length + num2.Length];
        for (int row = 0; row < num2.Length; row++)
        {
            for (int column = 0; column < num1.Length; column++)
            {
                result[column + row + 1] += rectangel[row][column];
            }
        }

        //对输出中数组进行进位运算
        for (int i = result.Length - 1; i >= 0; i--)
        {
            if (result[i] < 10)
                continue;

            result[i - 1] += result[i] / 10;
            result[i] %= 10;
        }
        string returnStr=new string(result.Select(i => (char)(i += '0')).ToArray());
        if(returnStr.StartsWith("0"))
        {