public class Category
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int ParentId { get; set; }
    }

    public class Product
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Categories { get; set; }
    }

    public class ProductWithThreeCate
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string AllCategoreis { get; set; }
    }

        static List<Category> GetCategories()
        {
            return new List<Category>()
            {
                new Category(){Id = 1, Name = "根", ParentId = -1},
                new Category(){Id = 2, Name = "一級(jí)分類1",ParentId = 1},
                new Category(){Id = 3, Name = "一級(jí)分類2", ParentId = 1},
                new Category(){Id = 4, Name = "二級(jí)分類11",ParentId = 2},
                new Category(){Id = 5, Name = "二級(jí)分類12",ParentId = 2},
                new Category(){Id = 6, Name = "二級(jí)分類21",ParentId = 3},
                new Category(){Id = 7, Name = "二級(jí)分類22",ParentId = 3},
                new Category(){Id = 8, Name = "三級(jí)分類111",ParentId = 4},
                new Category(){Id = 9, Name = "三級(jí)分類112",ParentId = 4},
                new Category(){Id = 10, Name = "三級(jí)分類121",ParentId = 5},
                new Category(){Id = 11, Name = "三級(jí)分類122",ParentId = 5},
                new Category(){Id = 12, Name = "三級(jí)分類211",ParentId = 6},
                new Category(){Id = 13, Name = "三級(jí)分類212",ParentId = 6},
                new Category(){Id = 14, Name = "三級(jí)分類221",ParentId = 7}
            };
        }

        static List<Product> GetProducts()
        {
            return new List<Product>()
            {
                new Product(){Id = 1, Name = "產(chǎn)品1",Categories = "10,12"},
                new Product(){Id = 2, Name = "產(chǎn)品2", Categories = "12,13"},
                new Product(){Id = 3, Name = "產(chǎn)品3",Categories = "10,11,12"},
                new Product(){Id = 4, Name = "產(chǎn)品4",Categories = "13,14"},
                new Product(){Id = 5, Name = "產(chǎn)品5",Categories = "11,13,14"}
            };
        }

        /// <summary>
        /// 獲取滿足某些條件的集合
        /// </summary>
        /// <param name="query">以英文逗號(hào)隔開的字符串，比如：2,5</param>
        /// <returns></returns>
        static List<ProductWithThreeCate> GetResultByQuery(string query)
        {
            //最終結(jié)果
            List<ProductWithThreeCate> result = new List<ProductWithThreeCate>();
            //臨時(shí)結(jié)果 此時(shí)ProductWithThreeCat的屬性AllCategoreis包含所有一級(jí)、二級(jí)、三級(jí)分類ID拼接成的字符串
            List<ProductWithThreeCate> tempResult = new List<ProductWithThreeCate>();
            //獲取所有的產(chǎn)品
            List<Product> allProducts = GetProducts();
            //遍歷這些產(chǎn)品
            foreach (var item in allProducts)
            {
                ProductWithThreeCate productWithThreeCate = new ProductWithThreeCate();
                productWithThreeCate.Id = item.Id;
                productWithThreeCate.Name = item.Name;
                //所有一級(jí)、二級(jí)、三級(jí)拼接成以英文逗號(hào)隔開的字符串
                string temp = string.Empty;
                //當(dāng)前產(chǎn)品只包含三級(jí)拼接成的、也是以英文隔開的字符串，split成數(shù)組
                string[] theThirdCates = item.Categories.Split(',');
                //遍歷這些三級(jí)數(shù)組
                foreach (string i in theThirdCates)
                {
                    //三級(jí)類別轉(zhuǎn)換成整型
                    int theThirdInt = int.Parse(i);
                    //獲取三級(jí)類別
                    Category theThirdCate = GetCategories().Where(c => c.Id == theThirdInt).FirstOrDefault();
                    //獲取二級(jí)類別
                    Category theSecondCate = GetCategories().Where(c => c.Id == theThirdCate.ParentId).FirstOrDefault();
                    //獲取一級(jí)類別
                    Category theFirstCate = GetCategories().Where(c => c.Id == theSecondCate.ParentId).FirstOrDefault();
                    temp += i + "," + theSecondCate.Id.ToString() + "," + theFirstCate.Id.ToString() + ",";
                }
                //去掉最后一個(gè)英文逗號(hào)
                temp = temp.Substring(0, temp.Length - 1);
                //轉(zhuǎn)換成集合，去除重復(fù)項(xiàng)，比如不同的三級(jí)可能有相同的一級(jí)或二級(jí)父類
                IEnumerable<string> tempArray = temp.Split(',').AsEnumerable().Distinct();
                //所有一級(jí)、二級(jí)、三級(jí)拼接成以英文逗號(hào)隔開的字符串，但已經(jīng)去除了重復(fù)的一級(jí)和二級(jí)
                string tempagain = string.Empty;
                //再次遍歷集合拼接成字符串
                foreach (var s in tempArray)
                {
                    tempagain += s + ",";
                }
                productWithThreeCate.AllCategoreis = tempagain.Substring(0, tempagain.Length - 1);
                tempResult.Add(productWithThreeCate);
            }
            //遍歷臨時(shí)結(jié)果
            foreach (var item in tempResult)
            {
                //把當(dāng)前包含一級(jí)、二級(jí)、三級(jí)的，以英文逗號(hào)隔開的字符串split成數(shù)組
                string[] itemArray = item.AllCategoreis.Split(',');
                //把當(dāng)前查詢字符串split成數(shù)組
                string[] queryArray = query.Split(',');
                //如果queryArray的每一個(gè)元素都被包含在itemArray中，那就保存起來(lái)
                if (queryArray.All(x => itemArray.Contains(x)) == true)
                {
                    result.Add(item);
                }
            }
            return result;
        }                

            List<ProductWithThreeCate> result = GetResultByQuery("2,5");
            //遍歷最終的結(jié)果
            foreach (var item in result)
            {
                Console.WriteLine(item.Name+ "  " + item.AllCategoreis);
            }
            Console.ReadKey();