Add Positive Integer Numbers

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

namespace _08.AddNumbers
{
    class Program
    {
        // Write a method that adds two positive integer numbers represented as arrays of digits
        // (each array element arr[i] contains a digit; the last digit is kept in arr[0]).
        // Each of the numbers that will be added could have up to 10 000 digits.

        static BigInteger addArraysOfDigits(int[] arr1, int[] arr2)
        {
            int maxLen = arr1.Length > arr2.Length ? arr1.Length:arr2.Length;

            int[] tempArr = new int[maxLen + 1];
            int[] sum = new int[maxLen + 1];

            if (arr1.Length > arr2.Length)
            {
                for (int i = 0; i < maxLen; i++)
                {
                    if (i < arr2.Length)
                    {
                        tempArr[i] = arr2[i];
                    }
                    else
                    {
                        tempArr[i] = 0;
                    }
                }

                for (int i = 0; i < sum.Length; i++)
                {
                    if (i < arr1.Length)
                    {
                        sum[i] = (arr1[i] + tempArr[i]) % 10;
                    }
                    if (i > 0 && arr1[i - 1] + tempArr[i - 1] >= 10)
                    {
                        sum[i] += 1;
                    }
                    //Console.Write(sum[i] + ", ");
                }
            }
            else
            {
                for (int i = 0; i < maxLen; i++)
                {
                    if (i < arr1.Length)
                    {
                        tempArr[i] = arr1[i];
                    }
                    else
                    {
                        tempArr[i] = 0;
                    }
                }

                for (int i = 0; i < sum.Length; i++)
                {
                    if (i < arr2.Length)
                    {
                        sum[i] = (arr2[i] + tempArr[i]) % 10;
                    }
                    if (i > 0 && arr2[i - 1] + tempArr[i - 1] >= 10)
                    {
                        sum[i] += 1;
                    }
                    //Console.Write(sum[i] + ", ");
                }
            }

            string resultString = "";

            for (int i = sum.Length - 1; i >= 0; i--)
            {
                resultString += sum[i];
            }

            BigInteger result = BigInteger.Parse(resultString);

            return result;
        }

        static void Main(string[] args)
        {
            int[] arr1 = {9, 1, 1, 1};
            int[] arr2 = {9, 2, 3, 9};

            BigInteger result = addArraysOfDigits(arr1, arr2);
            Console.WriteLine(result);
        }
    }
}