using System;

using System.Collections.Generic;

public class Program

{

​

    // Write a method which takes a string as an input and removes all characters which are duplicates in the string. Each character should only appear, at most, one time in the results (case insensitive).

    // Ex. This is a test -> This ae

     static char []RemoveDuplicateCharacters(char []str, int n) 

    {

         HashSet<char>s = new HashSet<char>(n - 1);

    foreach(char x in str)

        s.Add(x);

    char[] st = new char[s.Count];

    int i = 0;

    foreach(char x in s)

       st[i++] = x;

    return st;

        //Throw new Exception();

    }

        public static void Main()

    {

     char []str= "This is a tesTAa".ToCharArray();

   int n = str.Length;

   Console.Write(RemoveDuplicateCharacters(str, n));

    }

}