Amazon

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
  class App
  {
    public List<List<Item>> Nodes { get; set; }
    public Item Head { get; set; }

    public App()
    {
      Nodes = new List<List<Item>>();
      InitializeData();
    }

    public void Main()
    {
      ProcessNode(Head, 0);
      UpdateSiblings();
      DisplaySiblings();
      Console.ReadLine();
    }

    private void DisplaySiblings()
    {
      int level = 0;

      foreach (var itemList in Nodes)
      {
        level++;
        Console.Write($"Level {level}:");
        Console.WriteLine(string.Join(",", itemList.Select(x => x.Id.ToString()).ToArray()));
        DisplayLinkedListNode(itemList[0]);
        Console.WriteLine();
      }
    }

    private void DisplayLinkedListNode(Item aItem)
    {
      if(aItem != null)
      {
        Console.Write($"{aItem.Id}");
        if (aItem.next != null) Console.Write(",");
        DisplayLinkedListNode(aItem.next);
      }
     
    }

    private void UpdateSiblings()
    {
      foreach (var itemList in Nodes)
      {
        Item currentItem = null;
        foreach (var item in itemList)
        {
          if (currentItem == null)
          {
            currentItem = item;
          }
          else
          {
            currentItem.next = item;
            currentItem = item;
          }
        }
      }
    }

    private void ProcessChildren(Item aItem, int aLevel)
    {
      if (aItem != null)
      {
        ProcessNode(aItem.Left, aLevel + 1);
        ProcessNode(aItem.Right, aLevel + 1);
      }
    }

    private void ProcessNode(Item aItem, int aLevel)
    {
      if (aItem != null)
      {
        EnsureItemList(aLevel);
        Nodes[aLevel].Add(aItem);
        ProcessChildren(aItem, aLevel);
      }
    }

    private void EnsureItemList(int aLevel)
    {
      if (Nodes.Count <= aLevel)
      {
        Nodes.Add(new List<Item>());
      }
    }

    private void InitializeData()
    {
      Head = new Item()
      {
        Id = 1,
        Left = new Item()
        {
          Id = 2,
          Left = new Item()
          {
            Id = 3,
            Left = null,
            Right = new Item()
            {
              Id = 4,
              Left = null,
              Right = null
            }
          },
          Right = new Item()
          {
            Id = 5,
            Left = null,
            Right = null
          }
        },
        Right = new Item()
        {
          Id = 6,
          Left = null,
          Right = null
        }
      };
    }

  }

  public class Item
  {
    public int Id { get; set; }
    public Item Left { get; set; }
    public Item Right { get; set; }
    public Item next { get; set; }
  }

  public class Program
  {
    static void Main(string[] args)
    {
      new App().Main();
    }
  }
}