using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var numbers = new Dictionary<int,int[]>

        {

            {1, new [] { 2, 3, 4 }},

            {2, new [] { 5, 6, 7 }},

            {3, new [] { 8, 9, 10 }},

            {4, new [] { 11, 12, 13 }},

            {5, new [] { 14, 15, 16 }},

            {9, new [] { 17, 18, 19 }},

            {13, new [] { 20, 21, 22 }}

        };

        var demo = new Demo();

        Console.WriteLine(string.Join(", ", demo.GetList(numbers, 1)));

    }

}

​

public class Demo

{

    public IEnumerable<int> GetList(Dictionary<int,int[]> dict, int root)

    {

        return GetChildren(root);

        IEnumerable<int> GetChildren(int n) =>

            !dict.ContainsKey(n)

                ? new [] { n }

                : new [] { n }.Concat(dict[n].SelectMany(x => GetChildren(x)));

    }

}