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));
}