using System;

using System.Collections.Generic;

public class Program

{

    public static string ConvertNumToBinaryString(int num, int length)

    {

        string binary = Convert.ToString(num, 2);

​

        // добавляем ведущие нули до размера length

        // Например, length = 5, binary = 101 -> "00101"

        if (binary.Length < length) 

        {

            binary = string.Concat(System.Linq.Enumerable.Repeat("0", length - binary.Length)) + binary;

        }

​

        return binary;

    }

​

    public static string FindMissedBinaryString(string[] nums) {

        var set = new HashSet<string>(nums);

        var len = nums[0].Length;

        int digitNum = (int)Math.Pow(2, len);

​

        for (int i = 0; i < digitNum; i++)

        {

            string binary = ConvertNumToBinaryString(i, len);

            if (!set.Contains(binary)) 

            {

                return binary;

            }

        }

​

        return "";

    }

    public static void Main()

    {

        Console.WriteLine("UniLecs");

        // tests

        Console.WriteLine(FindMissedBinaryString(new string[] { "01", "10" })); // "00"

        Console.WriteLine(FindMissedBinaryString(new string[] { "000", "111","011","001" })); // "010"

    }

}