UVA 11105 Semi-prime H-numbers

https://vjudge.net/problem/UVA-11105

 

筛法

#include<cstdio>
#include<cstring>
#define N 1000002 
using namespace std;
int v[N],f[N/4+2];
int main()
{
    memset(v,-1,sizeof(v));
    for(int i=5;i<N;i+=4)
        for(int j=5;i*j<N;j+=4)
        {
            if(v[i]==-1 && v[j]==-1) v[i*j]=1;
            else v[i*j]=0;
        }
    int m=N/4,s=0;
    for(int i=1;i<m;i++)
    {
        if(v[i*4+1]==1) s++;
        f[i]=s;
    }
    int n;
    while(scanf("%d",&n)!=EOF) 
    {
        if(!n) return 0;
        printf("%d %d\n",n,f[n-1>>2]);
    }
}