using System;

using System.Collections.Generic;

public class Program

{

    static IEnumerable<Node> BFS(Node root)

    {

        yield return root;

​

        using var enumerator = BFS(root).GetEnumerator();

        foreach (var node in VisitLevel(root, enumerator, 1))

            yield return node;

    }

    static IEnumerable<Node> VisitLevel(Node root, IEnumerator<Node> enumerator, int nodeCount)

    {

        int childCount = 0;

​

        for (; nodeCount > 0; nodeCount--)

        {

            enumerator.MoveNext();

            foreach (var child in enumerator.Current.Children)

            {

                yield return child;

                childCount++;

            }

        }

​

        if (childCount > 0)

            foreach (var node in VisitLevel(root, enumerator, childCount))

                yield return node;

    }

    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; }

    }   

}