Euler P31 Solution

using System;
using System.Collections.Generic;
using System.Linq;

namespace Euler_P31
{
    class Program
    {
        static void Main()
        {
            Console.BufferHeight = 200;
            // Count the number of ways to make change for targetSum using the denominations in coinSet
            int[] coinSet = { 1, 2, 5, 10, 20, 50, 100, 200, 500, 1000, 2000 }; //, 10, 20, 50, 100 };
            int targetSum = 200;

            Console.WriteLine(CountSubsetSums(ref coinSet, targetSum));

        }

        public static ulong CountSubsetSums(ref int[] set, int targetSum)
        {
            Stack<int> myStack = new Stack<int>();
            int stackTotal = 0;
            int i = 0;
            ulong count = 0;

            int[] mySet = set.Where(g => g <= targetSum).ToArray();
            mySet = mySet.OrderByDescending(g => g).ToArray();
            bool modeAscending = true;
            while (true)
            {
                if (stackTotal >= targetSum)
                {
                    if (stackTotal == targetSum)
                    {
                        count++;
                        /*
                        foreach (int item in myStack)
                        {
                            Console.Write(item + ", ");
                        }
                        Console.WriteLine();
                        */
                    }

                    if (modeAscending || i != mySet.Length - 1)
                        i = Array.IndexOf(mySet, myStack.Peek()) + 1;
                    if (i == mySet.Length)
                        break;
                    while (myStack.Count != 0 && myStack.Peek() <= mySet[i]) // Depends on the order of the conditions
                    {
                        stackTotal -= myStack.Pop();
                    }

                    if (!modeAscending)
                        i = Array.IndexOf(mySet, myStack.Peek()) + 1;

                    stackTotal -= myStack.Pop();
                    if (i == mySet.Length - 1 && modeAscending)
                        modeAscending = false;
                }
                else
                {
                    if (stackTotal == 0)
                        modeAscending = true;
                    myStack.Push(mySet[i]);
                    stackTotal += mySet[i];
                }
            }
            return count;
        }
    }
}