Minimum sum of two numbers formed from digits of an array

using System;

class GfG{
    
    // Function to add two numeric strings and return the result as a string
    static string addString(string s1, string s2){
        
        int i = s1.Length - 1; 
        int j = s2.Length - 1; 
        int carry = 0;        
        string res = "";      
        
        // Loop until both strings are processed and no carry remains
        while (i >= 0 || j >= 0 || carry > 0){
            int sum = carry;

            // Add current digit from s1 if any
            if (i >= 0) sum += s1[i--] - '0';

            // Add current digit from s2 if any
            if (j >= 0) sum += s2[j--] - '0';

            // Add least significant digit of sum to result
            res = (sum % 10) + res;
            carry = sum / 10;
        }

        // Remove any leading zeros (except when result is "0")
        int k = 0;
        while (k < res.Length - 1 && res[k] == '0') k++;

        return res.Substring(k); 
    }

    // Function to find and return minimum possible sum of two numbers
    // formed by using all digits in the array
    static string minSum(int[] arr){
        
        // Sort the digits in ascending order
        Array.Sort(arr); 

        string s1 = "";  
        string s2 = "";  

        // Distribute digits alternately between s1 and s2
        for (int i = 0; i < arr.Length; i++)
        {
            if (i % 2 == 0)
                s1 += arr[i];
            else
                s2 += arr[i];
        }

        // Add the two numbers and return their sum as a string
        return addString(s1, s2);
    }

    static void Main(string[] args)
    {
        int[] arr = { 6, 8, 4, 5, 2, 3, 0 };
        Console.WriteLine(minSum(arr));  
    }
}

using System;
using System.Collections.Generic;

class GfG {

    // Function to add two strings and return the result
    static string addString(string s1, string s2) {
        
        int i = s1.Length - 1;
        int j = s2.Length - 1;

        // initial carry is zero
        int carry = 0;

        // we will calculate and store the
        // resultant sum in reverse order in res
        List<char> res = new List<char>();
        while (i >= 0 || j >= 0 || carry > 0) {
            int sum = carry;
            if (i >= 0)
                sum += (s1[i] - '0');

            if (j >= 0)
                sum += (s2[j] - '0');

            res.Add((char)(sum % 10 + '0'));
            carry = sum / 10;
            i--;
            j--;
        }

        // remove leading zeroes which are currently
        // at the back due to reversed string res
        while (res.Count > 0 && res[res.Count - 1] == '0')
            res.RemoveAt(res.Count - 1);

        // reverse our final string
        res.Reverse();

        return res.Count == 0 ? "0" : new string(res.ToArray());
    }

    // Function to find minimum sum using count array approach
    static string minSum(int[] arr) {
        
        // Count array to store frequency of each digit
        int[] count = new int[10];

        // Count occurrences of each digit
        foreach (int num in arr) {
            count[num]++;
        }

        // Two strings for storing the two minimum numbers
        string s1 = "";
        string s2 = "";

        // Flag to alternate between s1 and s2
        bool firstNum = true;

        // Traverse count array from 0 to 9
        for (int digit = 0; digit < 10; digit++) {

            // Add digit to appropriate string 
            // while it has occurrences
            while (count[digit]-- > 0) {
                if (firstNum) {

                    // Avoid leading zeros for both numbers
                    if (!(s1.Length == 0 && digit == 0))
                        s1 += digit.ToString();
                    firstNum = false;
                } else {
                    if (!(s2.Length == 0 && digit == 0))
                        s2 += digit.ToString();
                    firstNum = true;
                }
            }
        }

        // Handle case where s1 or s2 might be empty
        if (s1.Length == 0) s1 = "0";
        if (s2.Length == 0) s2 = "0";

        return addString(s1, s2);
    }

    static void Main() {
        int[] arr = { 6, 8, 4, 5, 2, 3, 0 };
        Console.WriteLine(minSum(arr));
    }
}