poj 3292 Semi-prime H-numbers

数论水题，利用筛选法……

#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<vector>
#include<cmath>
#include<iomanip>
#include<string>
using namespace std;
int an[1000001];
int main()
{
    int i,j,k,n;
    memset(an,0,sizeof(an));
    for(i=5;i<=1000000;i+=4)
    {
        for(j=5;j<=1000000;j+=4)
        {
            if(i*j>1000000)
                break;
            if(an[j]==0&&an[i]==0)
                an[i*j]=1;
            else an[i*j]=-1;
        }
    }
    k=0;
    an[1]=0;
    for(i=5;i<=1000000;i+=4)
        if(an[i]==1)
        {
            an[i]=++k;
        }
        else an[i]=k;
    while(cin>>n&&n)
    {
        cout<<n<<' '<<an[n]<<endl;
    }
    return 0;
}