using System;

using System.Collections.Generic;

using System.Linq;

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 zerosLength = l.TakeWhile(x => x == 0).Count();

        Enumerable.Repeat((int?)null, zerosLength).Concat(l.Skip(zerosLength)).Dump();

    }

}