素数筛法

生成MAX_N内的素数表

#define MAX_N XXXXXXX
#define MAX_PRIME MAX_N
vector primes;
vector is_prime;
void init_primes()
{
	is_prime = vector(MAX_PRIME + 1, true); //初始化为true
	is_prime[0] = is_prime[1] = false;
	for (int i = 2; i <= MAX_PRIME; ++i)
	{
		if (is_prime[i])   //优化
		{
			primes.push_back(i); 
			for (int j = i * 2; j <= MAX_PRIME; j += i)
			{
				is_prime[j] = false;   //将素数的倍数2倍、3倍等全都赋值为false
			}
		}
	}
}