HDU 2973 YAPTCHA

Description

给个公式，求结果
见HDU 2973

Algorithm

见威尔逊定理
然后就是如果3k+7是质数，就是1，否则就是0

Code

#include <cmath>
#include <iostream>
using namespace std;
const int maxn = 1e6+9;
int ans[maxn] = {0};
bool ok(int x)
{
  x = x * 3 + 7;
  for (int i = 2; i <= sqrt(x); i++)
    if (x % i == 0) return false;
  return true;
}
void init()
{
  for (int i = 1; i < maxn; i++)
  {
    ans[i] = ans[i - 1] + ok(i);
  }
}
int main()
{
  init();
  int t;
  cin >> t;
  for (int i = 0; i < t; i++)
  {
    int n;
    cin >> n;
    cout << ans[n] << endl;
  }
}