using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class HItem

    {

        public int Id {get; set;}

        public int? ParentId {get; set;}

    }

    public class Item

    {

        public int Id {get; set;}

        public List<Item> Items {get; set;}

        public Item(HItem item, Dictionary<int,List<HItem>> dict){

            Id = item.Id;

            if(dict.TryGetValue(item.Id, out var children))

                Items = children.Select(x => new Item(x, dict)).ToList();

        }

    }

    public static void Main()

    {

        var hItem1 = new HItem { Id = 1 };

        var hItem2 = new HItem { Id = 2, ParentId = 1 };

        var hItem3 = new HItem { Id = 3, ParentId = 1 };

        var hItem4 = new HItem { Id = 4, ParentId = 2 };

        var hItem5 = new HItem { Id = 5, ParentId = 4 };

        var hItem6 = new HItem { Id = 5 };

        var hItems = new List<HItem> { hItem1, hItem2, hItem3, hItem4, hItem5, hItem6 };

        var dhItems = hItems.GroupBy(x=>x.ParentId).ToDictionary(x => x.Key??0, x => x.ToList());

        foreach(var key in dhItems.Keys) Console.WriteLine(string.Format("Key {0}, Count {1}",key, dhItems[key].Count()));

        var rootHItems = dhItems[0];

        var items = rootHItems.Select(x => new Item(x, dhItems)).ToList();

    }

}