using System;

using System.Collections.Generic;

using System.Linq;

public class CategoriesMapping {

    public int CategoriesMappingID;

    public int CatID;

    public int MapCatID;

}

​

public class Program

{

      static List<CategoriesMapping> categoriesMapping = new List<CategoriesMapping>

        {

            new CategoriesMapping { CategoriesMappingID = 1, CatID = 1, MapCatID = 2},

            new CategoriesMapping { CategoriesMappingID = 2, CatID = 1, MapCatID = 5},

            new CategoriesMapping { CategoriesMappingID = 3, CatID = 2, MapCatID = 3},

            new CategoriesMapping { CategoriesMappingID = 4, CatID = 2, MapCatID = 4},

            new CategoriesMapping { CategoriesMappingID = 5, CatID = 5, MapCatID = 6},

            new CategoriesMapping { CategoriesMappingID = 6, CatID = 5, MapCatID = 7},

            new CategoriesMapping { CategoriesMappingID = 7, CatID = 5, MapCatID = 10},

            new CategoriesMapping { CategoriesMappingID = 8, CatID = 5, MapCatID = 11},

            new CategoriesMapping { CategoriesMappingID = 9, CatID = 2, MapCatID = 12},

            new CategoriesMapping { CategoriesMappingID = 10, CatID = 3, MapCatID = 13},

            new CategoriesMapping { CategoriesMappingID = 11, CatID = 3, MapCatID = 14}

        };

    public static void Main()

    {

        foreach(int i in GetNoDescendateCategoriesCategoryId(2).OrderBy(x=>x))

            Console.WriteLine("Category:" + i);

    }

    public static List<int> GetNoDescendateCategoriesCategoryId(int categoryId) {

       List<int> newCategories = new List<int>();

       List<int> childCategories = categoriesMapping.Where(m => m.CatID == categoryId).Select(x=> x.MapCatID).ToList();

​

       if(childCategories.Count == 0) {

          return new List<int>() {categoryId};

       }

       foreach (int catId in childCategories ) {

          newCategories.AddRange(GetNoDescendateCategoriesCategoryId(catId));

       }

       return newCategories;

   }

}