【CodeVS3223】素数密度

#include<iostream>
#include<cstring>
#include<cmath>
using namespace std;
bool pri[50000000];
bool dpri[1000010];

long long l,r,q,cnt,ans,q2;
int main()
{
	memset(pri,true,sizeof(pri));
	memset(dpri,true,sizeof(dpri));
	cin>>l>>r;
	pri[1]=false;
	for (int i=2;i<=ceil(sqrt(r));i++){
		if (pri[i]){
			q=i;
			while (q<ceil(sqrt(r))){
				q+=i;
				pri[q]=false;
			}
			for (q2=l;q2%i&&q2<r;q2++);
			while (q2<=i)q2+=i;
			while(q2<=r){
				dpri[q2-l]=false;
				q2+=i;
			}
		}
	}
	for (int i=l;i<=r;i++) if (dpri[i-l]) ans++;
	cout<<ans;
}