Перестановки. 9-й класс. Задача на четность

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

namespace ShaleNumbers
{
    public class ShaleSequence
    {
        private static readonly object syncRoot = new object();

        private int Length;
        private int[] Sequence;
        private byte[] Bucket;

        public ShaleSequence(int N, int a, int b)
        {
            Sequence = new int[N];
            Bucket = new byte[N];
            for (int i = 0; i < Bucket.Length; i++) Bucket[i] = 1;
            Add(a);
            Add(b);
        }

        public int N => Bucket.Length;

        public int[] ToArray() => Sequence.Take(Length).ToArray();

        public override string ToString() => String.Join(" ", ToArray());

        public bool IsFound => Length == Sequence.Length;

        private bool IsSet(int n) => Bucket[n - 1] == 1;

        private void Add(int x)
        {
            Sequence[Length++] = x;
            Bucket[x - 1] = 0;
        }

        private void Remove(int x)
        {
            Length--;
            Bucket[x - 1] = 1;
        }

        public void Scan()
        {
            Scan(Sequence[0], Sequence[1]);
        }

        private void Scan(int a, int b)
        {
            if (IsFound)
            {
                string info = ToString();
                lock (syncRoot) Console.WriteLine(info);
                return;
            }

            for (int c = a - (b % a); c <= N; c += a)
            {
                if (!IsSet(c)) continue;
                Add(c);
                Scan(b, c);
                Remove(c);
            }
        }

        public static void FindShaleSequences(int N)
        {
            DateTime start = DateTime.UtcNow;
            List<Task> tasks = new List<Task>();

            for (int a = 1; a <= N; a++)
            {
                for (int b = 1; b <= N; b++)
                {
                    if (a == b) continue;

                    ShaleSequence sequence = new ShaleSequence(N, a, b);
                    Task task = Task.Factory.StartNew(sequence.Scan);
                    tasks.Add(task);
                }
            }
            Task.WaitAll(tasks.ToArray());
            DateTime end = DateTime.UtcNow;
            Console.WriteLine($"N: {N}; Time: {(int)(end - start).TotalSeconds} sec");
        }

    }

    public class Program
    {
        public static void Main(string[] args)
        {
            for(int N = 3; N <= 100; N++)
            {
                ShaleSequence.FindShaleSequences(N);
            }
        }
    }
}