区间素数筛

#include <iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
using namespace std;
const int maxn=9*1e8;
int cnt;
bool p[maxn];
bool prime[maxn];
void is_prime(int n,int m)
{
    cnt=m-n;
    memset(p,true,sizeof(p));
    for(int i=0;i<cnt;i++) prime[i]=true;
    if(n<2)
    {
        prime[0]=false;
        cnt--;
        if(n==0)
            {
                prime[1]=false;
                cnt--;
            }
    }
    int e=floor(sqrt(m)+0.5);
    for(int i=2;i<=e;i++)
    {
        if(p[i]) {
                        int pos;
                        bool flag=false;
                        for(int j=i*i;j<=e;j+=i)
                            p[j]=false;
                        for(int j=n;j<m;j++)
                             if(!(j%i)&&j!=i) {
                                              pos=j;
                                              flag=true;
                                              break;
                                           }
                        if(flag) {
                                   for(int j=pos;j<m;j+=i)
                                        if(prime[j-n]) {
                                            cnt--;
                                            prime[j-n]=false;
                                        }
                                   }
                   }
    }
}
int main()
{
    int n,m;                //左闭右开的区间
    while(scanf("%d%d",&n,&m)==2&&m>n)
    {
        is_prime(n,m);
        printf("%d\n",cnt);
    }
    return 0;
}