using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var l = new List<int?>{0,0,0,0,0,0,4,5,6,3,0,0,2,3};

        var l2 = l.Select((x, i) => i == 0 || l[i - 1] == 0 ? (int?)null : x);

        Console.WriteLine(string.Join(",", l2));

    }

}