using System;

using System.Linq; 

public class Program

{

    public static void Main()

    {

        var testArray = new string[] {"??", "??", "FF", "5B", "??", "01", "??" };

        var r = testArray.SkipWhile(x=>x.Equals("??"))

                 .Reverse()

                 .SkipWhile(x=>x.Equals("??"))

                 .Reverse();

        Console.WriteLine(string.Join(",", r));

    }

}