求质数的算法

class Program
    {
        static void Main(string[] args)
        {
　　　　　　　//From 0 - Num
            int num = 10000000;
            PrimeNumber(num);
            //PrimeNumber2(num);
            PrimeNumberWithoutTwo(num);
            //PrimeNumberWithoutTwo2(num);

        }

        private static void PrimeNumber(int num)
        {
            int count = 0;
            bool isPrime;
            for (int i = 2; i <= num; i++)
            {
                isPrime = true;
                for (int j = 2; j < i / j + 1; j++)
                //for (int j = 2; j < i; j++)
                {
                    if (i % j == 0)
                        isPrime = false;
                    count++;
                }
                //if (isPrime)
                //    Console.WriteLine(i);

            }
            Console.WriteLine("total count:{0:d}", count);
        }

        private static void PrimeNumberWithoutTwo(int num)
        {
            int count = 0;
            bool isPrime;
            if (num % 2 == 0)
                num = num - 1;
            for (int i = 3; i <= num; i = i + 2)
            {
                isPrime = true;
                for (int j = 3; j < i / j + 1; j = j + 2)
                //for (int j = 2; j < i; j++)
                {
                    if (i % j == 0)
                        isPrime = false;
                    count++;
                }
                //if (isPrime)
                //    Console.WriteLine(i);

            }
            Console.WriteLine("total count:{0:d}", count);
        }
}