求质数：普通筛选法

const int N = 1000000;

int main() {
	vector<bool> v;
	v.resize(N, 1);
	v[0] = v[1] = 0;
	for (int i = 2;i<N; i++) {
		if (v[i]) {
			for (int j = 2 * i; j < N; j += i) {
				v[j] = 0;
			}
		}
	}
	return 0;
}