HackerRank world code sprint #6 - BrontTrousle - try to shorten the time - failed, still score 0/50, only pass 1 test case

BroneTrousle_2.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace boneTrousle
{
    public class Node
    {
        public string purchaseBoxes;
        public int length;
        public int boxCount;
        public long sum; 
    }


    class Program
    {
        static void Main(string[] args)
        {
            int queries = Convert.ToInt32(Console.ReadLine().Trim());

            for (int i = 0; i < queries; i++)
            {
                string[] arr = Console.ReadLine().Split(' ');

                long total = Convert.ToInt64(arr[0]);
                long storeKBox = Convert.ToInt64(arr[1]);
                int purchaseBoxCount = Convert.ToInt32(arr[2]);

                IList<string> list = new List<string>();               

                buildResult_usingDFS_stack(total, storeKBox, purchaseBoxCount, list);  // think about design here

                string res = "-1";
                
                if (list.Count > 0)
                {
                    string[] arr2 = list[0].Split(';');

                    res = "";
                    bool skipFirst = true;
                    foreach (string s in arr2)
                    {
                        if (skipFirst)
                        {
                            res += s;
                            skipFirst = false;
                        }
                        else
                        {
                            res += " " +s;
                        }
                    }
                }

                Console.WriteLine(res); 
            }
        }
        /*
         * source code reference:
         * https://gist.github.com/jianminchen/872bf70039fa8c61ff208b34a591c8ec
         * 
         */
        public static void buildResult_usingDFS_stack(long total, long storeKBox, int purchaseBoxCount, IList<string> list)
        {           
            // need to complete the task using Queue
            if (purchaseBoxCount == 0 || storeKBox == 0)
                return;
            if(purchaseBoxCount > storeKBox/2)
            {
                long rest = sum(storeKBox);
                if (total > rest)
                    return;
                rest -= total;
                buildResult_usingDFS_stack(rest, storeKBox, (int)(storeKBox - purchaseBoxCount), list);
                return; 
            }

            Stack<Node> stack = new Stack<Node>();

            int[] arr = new int[2] { 0, 1 }; 
            foreach (int val  in arr)
            {
                Node node = new Node();

                node.length = 1;
                if (val != 0)
                {
                    node.purchaseBoxes = val.ToString();
                    node.boxCount = 1;
                    node.sum = 1; 
                }
                else
                {
                    node.purchaseBoxes = "";
                    node.boxCount = 0; 
                    node.sum = 0; 
                }

                stack.Push(node);
            }

            while (stack.Count > 0)
            {
                Node node = (Node)stack.Pop();

                string s1    = node.purchaseBoxes;
                int len      = node.length;
                int boxCount = node.boxCount;
                long sum     = node.sum; 

                if (boxCount == purchaseBoxCount)
                {
                    if (sum == total)
                    {
                        list.Add(s1);
                        return; 
                    }
                }
                else if(len + 1 <= storeKBox)
                {
                    int[] tmpArr = new int[2] { 0, len + 1 };
                    foreach (int val in tmpArr)
                    {
                        Node newNode = new Node();

                        newNode.purchaseBoxes = s1 + (val==0?"":(";"+val.ToString())) ;
                        newNode.length        = len +1;
                        newNode.boxCount      = val==0? boxCount : (boxCount +1);
                        newNode.sum           = val ==0? sum : (sum + val); 

                        stack.Push(newNode);
                    }
                }
            }

            return;
        }    
    
        public static long sum(long n)
        {
            return (long)(n * (n + 1) / 2); 
        }
    }
}