Interview Question for Software Development Engineer Intern at Microsoft:

using System;
using System.Linq;

namespace ReverseString
{
    class Program
    {
        static void Main()
        {
            string s = "abcdxdd";

            int reverseEnd = s.IndexOf('x');
            if (reverseEnd < 0)
                reverseEnd = 0;

            string subAr1 = new string(s.Substring(0, reverseEnd).Reverse().ToArray());
            string subAr2 = s.Substring(reverseEnd);
            s = String.Concat(subAr1, subAr2);

            Console.WriteLine(s);
        }
    }
}