poj 3292（筛素数法变形）

#include<iostream>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<algorithm> 
#include<ctime>
#define ll long long
using namespace std;
const int maxn = 1000005;
int data[maxn],data2[maxn];
bool flag[maxn];
int init(int n){
    memset(flag,1,sizeof(flag));
    int i,j,k=0;
    int s,e = 1001;
    for(i=10;i<n;i+=5)flag[i] = 0;
    for(i=9;i<e;i+=4){
        if(flag[i]){
            data[k++] = i;
            for(s=i*2,j=i*i;j<n;j+=s)flag[j] = 0;
        }
    }
    for(;i<n;i+=4)if(flag[i])data[k++] = i;
    return k;
}
int isprime(int n){
    if(n%4!=1)return false;
    for(int i=5;i<=sqrt(1.0*n);i+=4){
        int t = n/i;
        if((n==t*i)&&(t%4==1)&&(flag[t]==1)&&(flag[i]==1)){
            return true;
        }
    }
    return false;
}
int main(){
    int k = init(maxn-1);
    int n,i,p=0;
    for(i=25;i<=maxn-4;i++){
        if(isprime(i)){
            data2[p++] = i;
        }
    }
    while(scanf("%d",&n)==1&&n){
        for(i=0;i<p;i++)
            if(data2[i]>n){
                break;
            }
        printf("%d %d\n",n,i);
    }
    return 0;
}