using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var node1 = new Node<int>(null,null,0);

        var node2 = new Node<int>(null,null,1);

        var node3 = new Node<int>(node1,node2,3);

        var node4 = new Node<int>(node3,null,4);

        var sum = 0;

        foreach(var value in node4){

            sum+=value;

        }

        Console.WriteLine(sum);

    }

}

​

public class Node<T> : IEnumerable<T>{

    private Node<T> leftNode;

    private Node<T> rightNode;

    private T value;

​

    public Node(Node<T> leftNode,Node<T> rightNode,T value){

        this.leftNode = leftNode;

        this.rightNode = rightNode;

        this.value = value;

    }

​

    public IEnumerator<T> GetEnumerator()

    {

        if(this.leftNode!=null){

            foreach(var value in this.leftNode){

                yield return value;

            }

        }

        yield return this.value;

        if(this.rightNode!=null){

            foreach(var value in this.rightNode){

                yield return value;

            }

        }

    }

        IEnumerator IEnumerable.GetEnumerator(){

            return this.GetEnumerator();

        }

}