using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(IsUnique("abc"));

    }

    public static bool IsUnique(string s)

    {

        var hash = new bool[256];

        var letters = s.ToCharArray();

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

        {

            var letter = letters[i];

            if(hash[letter] == false)

            {

                hash[letter] = true;

            }

            else

                return false;

        }

        return false;

    }

}