using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var root = new Node("A");

        var b = root.AddChild("B");

        root.AddChild("C");

        var d = root.AddChild("D");

        b.AddChild("E");

        var f = b.AddChild("F");

        var g = d.AddChild("G");

        d.AddChild("H");

        f.AddChild("I");

        f.AddChild("J");

        g.AddChild("K");

        var answer = new List<string>();

        answer = root.BreadthFirstSearch(answer);

        Console.Write("[");

        for(var i = 0; i < answer.Count; i++)

        {

            Console.Write(answer[i]);

            if (i < answer.Count - 1) Console.Write(", ");

        }

        Console.Write("]");

        //Sample output: ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K"]

    }

}

​

public class Node

{

    private string _name;

    private List<Node> _children;

​

    public Node(string name)

    {

        this._name = name;

        this._children = new List<Node>();

    }

    public Node AddChild(string name)

    {

        var newNode = new Node(name);

        this._children.Add(newNode);

        return newNode;

    }

    public List<string> BreadthFirstSearch(List<string> result)

    {

        var queue = new Queue<Node>();

        queue.Enqueue(this);

        while (queue.Count > 0)

        {

            var node = queue.Dequeue();

            result.Add(node._name);

            foreach (var child in node._children)

            {

                queue.Enqueue(child);

            }

        }

        return result;

    }

}

​

​