לפניך התוכנית הבאה.

public class Ex
{
    public static int[] Secret(char[] data)
    {
        int[] temp = new int[26];
        for (int i = 0; i < temp.Length; i++)
            temp[i] = 0;
        for (int k = 0; k < data.Length; k++)
        {
            int ind = (int)data[k] - 65;
            temp[ind]++;
        }
        return temp;
    }

    public static void Main(string[] args)
    {
        char[] let={'A','F','B','B','F','K','A','A','D','F'};
        int[] res = Secret(let);
        for (int i = 0; i < res.Length; i++)
        {
            int t = res[i];
            if (t > 0)
            {
                int code = i + 65;
                char ch = (char)code;
                for (int k = 1; k <= t; k++)
                    Console.Write(ch);
                Console.WriteLine();
            }
        }
    }
}