埃氏筛选法O(n)

const int maxn = 1e8 + 1;

int book[maxn];

void prime(int n) {
	memset(book, true, sizeof book);
	book[1] = false;
	for(int i = 2; i <= sqrt(n); ++i) {
		if(book[i]) {
			for(int j = 2; j <= n / i; ++j) {
				book[i * j] = false;
			}
		}
	}
}