[C++] 백준 4948번 : 베르트랑 공준

https://www.acmicpc.net/problem/4948

#include <iostream>

#include <math.h>

using namespace std;

int main() {

cin.tie(NULL);

ios::sync_with_stdio(false);

int ary[246913] = {0,};

int flag = 0;

ary[0]=0;

ary[1]=0;

for (int i = 2; i <= 246912; i++) {

for (int j = 2; j <= sqrt(i); j++)

{

if (i%j == 0)

{

flag = 1;

break;

}

}

if (flag == 1)

ary[i] = 1;

flag = 0;

}

while (1) {

int n;

cin >> n;

if (n == 0)

break;

int cnt = 0;

for (int i = n + 1; i <= 2 * n; i++) {

if(ary[i]==0)

cnt++;

}

cout << cnt << "\n";

}

}