using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var categoryArrays = new List<List<string>> {

            new List<string> {"one-one", "one-two"},

            null,

            new List<string> {"two-one"},

            new List<string> {"three-one", "three-two", "one-two"},

            new List<string> {"two-one"}

        };

        var categoriesList = categoryArrays.Where(a => a != null && a.Any()).SelectMany(a => a).Distinct().OrderBy(c => c).ToList();

        Console.WriteLine($"{categoriesList.Count} categories");

    }

}