http://hihocoder.com/contest/hiho93/problem/1

存一下尤拉篩模板

#include <bits/stdc++.h>

using namespace std;
#define LL long long
const int Mod = 1e9 + 7;
const int maxn = 1e6 + 5;
const double eps = 0.00000001;
const int INF = 0x3f3f3f3f;

bool isPrime[maxn];
int Cnt[maxn];
int prime[maxn];

void Isprime(
 
) {
    int counter = 0;
    memset(isPrime, true, sizeof(isPrime));
    for (int i = 2; i < maxn; i ++) {
        if(isPrime[i]) prime[++counter] = i;
        Cnt[i] = counter;
        for (int j = 1; j <= counter; j ++) {
            if(i * prime[j] > maxn)  break;
            isPrime[i * prime[
 
j]] = false;
            if(i % prime[j] == 0) break;
        }
    }
}

int main()
{
    Isprime();
    int n;
    while(cin >> n)  cout << Cnt[n] << endl;
    
    return 0;
}