Reverse And Shift Char in String, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Globalization;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CultureInfo ci = new CultureInfo("hi-IN");
            var numbers = new List<string>();
            numbers.Add(11223344556677889900.ToString("N0",ci));
            numbers.Add(10000000000000000000.ToString("N0",ci));
            for(var i = 0; i < numbers.Count; i++){
                numbers[i] = ShiftString(numbers[i]);
                Console.WriteLine((numbers[i]));
            }
            Console.Write(ReverseString(numbers[0]));
        }
        
        public static string ShiftString(string t)
        {
            string result = string.Empty;
            bool comma = false;
            foreach(var chr in t){
                if(chr == ','){
                    comma = true;
                    continue;
                }
                result += chr+(comma ? "," : "");
                comma = false;
            }
            return result;
        }
        
        ///Recursion method; simple and regular performance for small strings
        public static string ReverseString(string str)
        {
           if (str.Length <= 1)
            return str;
           else
           return ReverseString(str.Substring(1)) +str[0];
        }
    }
}

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Compilation time: 0,17 sec, absolute running time: 0,11 sec, cpu time: 0,09 sec, average memory usage: 12 Mb, average nr of threads: 3