using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(firstNotRepeatingCharacter("aaabbcsadadc"));

    }

    static char firstNotRepeatingCharacter(string s) {

   // idea is simple, first check uniqueness

   // if char is Unique set the bit, if 

   // it is not unset the bit

   // at the end find the first charcter for which the bit is set

   // first unique charcter. 

    int flag = 0;

    int uniqueCharcters = 0; 

    for(int i = 0; i< s.Length ; i++) {

        int uniqueBit = 1 << ((int)s[i]-97);

        if((uniqueBit & flag) > 0) {

            //non unique

            uniqueCharcters = uniqueCharcters & ~uniqueBit;

        }

        else {

            //unique

            uniqueCharcters = uniqueCharcters | uniqueBit;

        }

        flag = flag | uniqueBit;                       

    }

//  Console.WriteLine(Convert.ToString(uniqueCharcters,2));

    int j = -1; //index of first set bit

    while(uniqueCharcters!=0)

    {

        j++;

        if((uniqueCharcters&1) == 1)

            break;

        uniqueCharcters = uniqueCharcters >> 1;

    }

        return j==-1? '_':(char)(j+97);

    }

}

​

​