10.TextProcessingExercise-Multiply Big Number Like A String

10.Multiply Big Number
You are given two lines – the first one can be a really big number (0 to 10Pow50). The second one will be a single digit number (0 to 9). You must display the product of these numbers.
Note: do not use the BigInteger class.
Examples
Input                                Output
23                                   46
2

9999                                 89991
9

923847238931983192462832102          3695388955727932769851328408
4

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        var bigInteger = Console.ReadLine().TrimStart(new char[] {'0'}).ToCharArray();//Ако е 0000005005 ще стане '5''0''0''5'!
            var singleDigit = int.Parse(Console.ReadLine());
            if (singleDigit == 0)
            {
                Console.WriteLine(0);
                return;
            }
            var result = new List<char>();      //Вместо char,може да е string.
            var multiplyByOneDigit = "";        //Вместо "" ще е 0.
            var onePlusMind = 0;
            for (int i = bigInteger.Length - 1; i >= 0; i--)
            {
              multiplyByOneDigit = (singleDigit * int.Parse(bigInteger[i].ToString()) + onePlusMind).ToString();//Без .ToString()
              if (singleDigit * int.Parse(bigInteger[i].ToString()) + onePlusMind > 9)
              {
                onePlusMind = int.Parse(multiplyByOneDigit[0].ToString());//Тогава onePlusMind ще е onePlusMind /= 10;
                result.Insert(0, multiplyByOneDigit[1]);//И също вместо multiplyByOneDigit[1] ще е (multiplyByOneDigit %10).ToString()
                if (i == 0)
                {
                   result.Insert(0, onePlusMind.ToString()[0]);//Без .ToString()[0]
                }
              }
              else
              {
                 onePlusMind = 0;
                 result.Insert(0, multiplyByOneDigit[0]);//Без [0];
              }
           }
            Console.WriteLine(string.Join("", result));
    }
}