using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{   

    public class DistributionGroup

    {

        public string Id { get; set; }

        public List<string> Users { get; set; }

        public List<string> DistributionGroups { get; set; }

    }

    public static void Main()

    {

        Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(GetFlattenedListOfUserIds("d69c80c8-bc48-11ec-a3df-00155d5b564d")));

        // a5dca2a4-bc4a-11ec-b3af-00155d5b564d

        // ab2f6ca0-bc4a-11ec-8363-00155d5b564d

        // 5626e1c4-bc4b-11ec-9449-00155d5b564d

        // 86c027b4-bc4b-11ec-98f1-00155d5b564d

    }

    public static List<string> GetFlattenedListOfUserIds(string rootDistributionGroupId)

    {

        // Using a BFS search

        var users = new HashSet<string>();

        var visitedDistributionGroups = new HashSet<string>();

        var distributionGroupsToVisit = new Queue<string>();

        distributionGroupsToVisit.Enqueue(rootDistributionGroupId);

​

        while (distributionGroupsToVisit.Any())

        {

            var distributionGroupId = distributionGroupsToVisit.Dequeue();

            if (!visitedDistributionGroups.Contains(distributionGroupId))

            {

                var distributionGroup = GetByDistributionGroupId(distributionGroupId);

                // will be null if searching for a deleted user

                if (distributionGroup != null)

                {

                    users.UnionWith(distributionGroup.Users);

                    distributionGroup.DistributionGroups.ForEach(d => distributionGroupsToVisit.Enqueue(d));

                }

​

                visitedDistributionGroups.Add(distributionGroupId);

            }

        }

​

        return users.ToList();

    }

    // Mock the database call

    public static DistributionGroup GetByDistributionGroupId(string distributionGroupId)

    {

        Dictionary<string, DistributionGroup> distributionGroup = new()

        {

            { "d69c80c8-bc48-11ec-a3df-00155d5b564d", new() { Id = "d69c80c8-bc48-11ec-a3df-00155d5b564d", Users = new() { "a5dca2a4-bc4a-11ec-b3af-00155d5b564d", "ab2f6ca0-bc4a-11ec-8363-00155d5b564d" }, DistributionGroups = new() { "a5dca2a4-bc4a-11ec-b3af-00155d5b564d", "ab2f6ca0-bc4a-11ec-8363-00155d5b564d" } } },

            { "a5dca2a4-bc4a-11ec-b3af-00155d5b564d", new() { Id = "a5dca2a4-bc4a-11ec-b3af-00155d5b564d", Users = new() { "a5dca2a4-bc4a-11ec-b3af-00155d5b564d", "5626e1c4-bc4b-11ec-9449-00155d5b564d" }, DistributionGroups = new() { "d69c80c8-bc48-11ec-a3df-00155d5b564d" } } },

            { "ab2f6ca0-bc4a-11ec-8363-00155d5b564d", new() { Id = "ab2f6ca0-bc4a-11ec-8363-00155d5b564dd", Users = new() { }, DistributionGroups = new() { "659ab6da-bc4b-11ec-a6c0-00155d5b564d" } } },

            { "659ab6da-bc4b-11ec-a6c0-00155d5b564d", new() { Id = "ab2f6ca0-bc4a-11ec-8363-00155d5b564dd", Users = new() { "86c027b4-bc4b-11ec-98f1-00155d5b564d" }, DistributionGroups = new() { } } },

        };

        return distributionGroup[distributionGroupId];

    }

}