using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

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

        {

            [1] = new[] { 2 },

            [2] = new[] { 3, 5, 7, 9 },

            [3] = new[] { 4 },

            [4] = new[] { 5 },

            [5] = new[] { 6 },

            [6] = new[] { 7, 9 },

            [7] = new[] { 8 },

            [8] = new[] { 9 }

        };

​

        var output = input.GetPossibleRanges();

​

        foreach (var entry in output)

        {

            Console.WriteLine(string.Join(",", entry));

        }

    }

}

​

public static class Helpers

{

    public static IEnumerable<List<int>> GetPossibleRanges(this Dictionary<int, int[]> dict)

    {

        var ranges = new List<int> { dict.Keys.Min() }.BuildRanges(dict);

​

        return ranges;

    }

​

    public static IEnumerable<List<int>> BuildRanges(this List<int> range, Dictionary<int, int[]> dict)

    {

        if (dict.TryGetValue(range.Last(), out int[] nextLevel))

        {

            foreach (var child in nextLevel)

            {

                foreach (var grownRange in range.Append(child).ToList().BuildRanges(dict))

                {

                    yield return grownRange;

                }

            }

        }

        else

        {

            yield return range;

        }

    }   

}

​