51nod 1181 质数中的质数（质数筛法）

题目链接：51nod 1181 质数中的质数（质数筛法）

#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
const int N = 1000001;
int prime[N+1];
void getPrime(){
    memset(prime, 0, sizeof(prime));
    for(int i = 2;i <= N; i++){
        if(!prime[i])
            prime[ ++prime[0] ] = i;
        for(int j = 1; j <= prime[0] && prime[j] <= N / i; j++){
            prime[ prime[j] * i ] = 1;
            if(i % prime[j] == 0) break;
        }
    }
}
int main(){
    getPrime();
    int i, n, f;
    scanf("%d", &n);
    f = upper_bound(prime+1, prime + 1 + prime[0], n-1) - prime;
    f = upper_bound(prime+1, prime + 1 + prime[0], f-1) - prime;
    printf("%d\n", prime[prime[f]]);
    return 0;
}