Methods

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace Multiplication
{
    class Multiplication
    {
        static void Main(string[] args)
        {
            BigInteger num1 = BigInteger.Parse(Console.ReadLine());
            BigInteger num2 = BigInteger.Parse(Console.ReadLine());
            List<int> finalResult = MultiplyBigNumbers(num1, num2);
            PrintList(finalResult);
        }

        static List<int> MultiplyBigNumbers(BigInteger num1, BigInteger num2)
        {
            //make the smaller number to be first
            Swap(ref num1, ref num2);
            List<int> list1 = ConvertBigIntToList(num1);
            List<int> list2 = ConvertBigIntToList(num2);
            //create a list of lists with the multiplication of the digits of the shorter number with the longer number
            List<List<int>> helpList = new List<List<int>>();
            for (int i = 0; i < list1.Count; i++)
            {
                helpList.Add(ListAndNumber(list2, list1[i]));
            }

            AppendZeros(helpList);

            List<int> finalResult = new List<int>();
            int digit = 0;
            int carry = 0;
            for (int i = 0; i < helpList[helpList.Count - 1].Count; i++)
            {
                int product = carry;
                for (int j = 0; j < helpList.Count; j++)
                {
                    product += helpList[j][i];
                }
                if (product >= 10)
                {
                    digit = product % 10;
                    carry = product / 10;
                }
                else
                {
                    digit = product;
                    carry = 0;
                }
                finalResult.Add(digit);
            }
            finalResult.Add(carry);

            return finalResult;

        }

        static void AppendZeros(List<List<int>> helpList)
        {
            int zeros = helpList.Count - 1;
            for (int i = 0; i < helpList.Count; i++)
            {
                for (int j = 0; j < zeros; j++)
                {
                    helpList[i].Add(0);
                }
                for (int j = 0; j < i; j++)
                {
                    helpList[i].Insert(0, 0);
                }

                if (zeros > 0)
                {
                    zeros--;
                }
                else
                {
                    break;
                }
            }
        }

        //multiplies an array of ints and a single digit
        static List<int> ListAndNumber(List<int> list, int num)
        {
            List<int> result = new List<int>();
            int product = 0;
            int carry = 0;
            int digit = 0;
            for (int i = 0; i < list.Count; i++)
            {
                product = list[i] * num + carry;
                if (product >= 10)
                {
                    digit = product % 10;
                    carry = product / 10;
                }
                else
                {
                    digit = product;
                    carry = 0;
                }
                result.Add(digit);
            }
            result.Add(carry);
            return result;
        }

        //extract the digits of a number into an array;
        static List<int> ConvertBigIntToList(BigInteger n)
        {
            List<int> digits = new List<int>();
            while (n > 0)
            {
                digits.Add((int)(n % 10));
                n /= 10;
            }
            return digits;
        }

        //Print a list of digits in the following order: first prints the last element if other than 0. The last element it prints is also the last digit of the number.
        static void PrintList(List<int> list)
        {
            if (list[list.Count - 1] != 0)
            {
                Console.Write(list[list.Count - 1]);
            }
            for (int i = list.Count - 2; i >= 0; i--)
            {
                Console.Write(list[i]);
            }

            Console.WriteLine();
        }

        //A method to set the smaller number to be first
        static void Swap(ref BigInteger a, ref BigInteger b)
        {
            if (a > b)
            {
                a = a + b;
                b = a - b;
                a = a - b;
            }
        }
    }
}