using System;

using System.Collections.Generic;

public class Program

{

    static IEnumerable<Node> BFS(Node root)

    {

        yield return root;

​

        foreach (var n in BFS(root))

            foreach (var c in n.Children)

                yield return c;

    }

    static void Main()

    {

        var tree = new Node(1,

            new Node(2,

                new Node(5),

                new Node(6,

                    new Node(8))),

            new Node(3,

                new Node(7,

                    new Node(9),

                    new Node(10))),

            new Node(4));

​

        foreach (var node in BFS(tree))

            Console.WriteLine(node.Value);

    }

    class Node

    {

        public Node(int value, params Node[] children)

        {

            Value = value;

            Children = children;

        }

​

        public int Value { get; }

        public Node[] Children { get; }

    }   

}