List sort according to desired condition

var non_community1 =new List<string>{ "red-11","red-10","red-9","orange-11","green-11","green-7","green-9", "green-9" ,"orange-9","green-11"};

non_community1 = non_community1.OrderByDescending(s => int.Parse(Regex.Match(s, @"\d+").Value)).ToList();

for (int i = non_community1.Count - 1; i > 0; i--) 
{
    if ((non_community1[i] == non_community1[i - 1]))
    {   
        non_community1.RemoveAt(i);
    }
}

this code give me that to sort

this is the list i want to produce

I’m a little stuck at this part, how do I get out of it?

I want to write the same at least 3 numbers in different color groups in sequence and I want to add "null" between the numbers that write 3 or more consecutive numbers.

>Solution :

ok so (after you removed the duplicates) you would need to use GroupBy to aggregate the items with the same integer and sort them according to their occurence:

var groupedResult = non_community1
                        .GroupBy( s => int.Parse(Regex.Match(s, @"\d+").Value))
                        .OrderByDescending(k => k.Count()).ToList();

Now you need to go through each group and collect them in the final list.

and I want to add "null" between the numbers that write 3 or more consecutive numbers.

For this you can check how many items are in each group if you have more than 2 items then add the null item.

List<string> finalList = new List<string>();

for ( int i = 0; i < groupedResult.Count; i++ )
{
    finalList.AddRange(groupedResult[i]);
    if(groupedResult[i].Count() > 2)
    {
        finalList.Add(null);
    }
}

Console.WriteLine(string.Join(Environment.NewLine, finalList));

and you should have your desired result