/// <summary>

    /// 高效判断是否为质数

    /// </summary>

    public static bool isPrime(long num)

    {

        if (num < 2)

            return false;

        if (num == 2 || num == 3)

        {

            return true;

        }

        if (num % 6 != 1 && num % 6 != 5)

        {

            return false;

        }

        int sqr = (int)Math.Sqrt(num);

        for (int i = 5; i <= sqr; i += 6)

        {

            if (num % i == 0 || num % (i + 2) == 0)

            {

                return false;

            }

        }

        return true;

    }