using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

            var result = getShortestUniqueString(new char[] { 'x', 'y', 'z' }, "yxyyzyzxzzyx");

            Console.WriteLine(result);

    }

​

    static string getShortestUniqueString(char[] arr, string str)

    {

        var used = new Queue<char>(arr.Length);

        var rest = arr.ToList();

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

        {

            if (rest.Contains(str[i]))

            {

                used.Enqueue(str[i]);

                rest.Remove(str[i]);

            }

            else

            {

                char ch;

                while ((ch = used.Dequeue()) != str[i])

                    rest.Add(ch);

                used.Enqueue(str[i]);

            }

            if (rest.Count == 0)

                return str.Substring(i + 1 - arr.Length, arr.Length);

        }

        return string.Empty;

    }

}