using System;

using System.Collections.Generic;

​

public class Program

{

    // first repeated character from a string

    public static char GetFirstRepeatedCharacter(string str)

    {

        var hashSet = new HashSet<char>();

        foreach(var c in str)

        {

            if(!hashSet.Add(c))

                return c;

        }

        return '\0';

    }

    public static void Main()

    {

        Console.WriteLine(GetFirstRepeatedCharacter("abcdef_c"));

    }

}