using System;

public class Node

{

    public Node[] Children {get; set;}

    public Node Right {get;set;}

}

​

public class Program

{

    public static void Main()

    {

        var rootNode = new Node();

        PopulateTree(rootNode);

        PopulateRightNode(rootNode, null);

        Console.WriteLine(rootNode.Children[0].Right != null);

        Console.WriteLine(rootNode.Children[0].Right.Right != null);

        Console.WriteLine(rootNode.Children[0].Right.Right.Right == null);

    }

    private static void PopulateTree(Node node)

    {

        node.Children = new Node[] {

            new Node {

                Children = new Node[] {

                    new Node(),

                    new Node()

                }

            },

            new Node(),

            new Node {

                Children = new Node[] {

                    new Node()

                }

            }

        };

    }

    private static void PopulateRightNode(Node node, Node parent)

    {

        if (parent != null)

        {

          var index = -1;

          foreach(var child in parent.Children)

          {

              index++;

              if (child == node && index < parent.Children.Length - 1)

              {

                  node.Right = parent.Children[index + 1];

                  PopulateRightNode(node.Right, parent);

              }

              if (child == node && index == parent.Children.Length - 1)

              {

                  var startNode = parent.Right;

                  while(startNode != null)

                  {

                      if (startNode.Children == null && startNode.Right != null)

                      {

                          startNode = startNode.Right;

                          continue;

                      }

                      node.Right = startNode.Children[0];

                      PopulateRightNode(node.Right, startNode);

                  }

              }

          } 

        }

        else {

            PopulateRightNode(node.Children[0], node);

        }

    }

}