一道有点意思的逻辑题

public class ChildThreeDel
    {
        public static void main(String[] args)
        {
            bool[] arr = new bool[500];
            for (int i = 0; i < arr.Length; i++)
            {
                arr[i] = true;
            }
            int _count = arr.Length;
            int countNum = 0;
            int index = 0;
            while (_count > 1)
            {
                if (arr[index] == true)
                {
                    countNum++;
                    if (countNum == 3)
                    {
                        countNum = 0;
                        arr[index] = false;
                        leftCount--;
                    }
                }
                index++;
                if (index == arr.Length)
                {
                    index = 0;
                }
            }
            for (int i = 0; i < arr.Length; i++)
            {
                if (arr[i] == true)
                {
                    Console.WriteLine(i+1);
                }
            }
            Console.Read();
        }
    }