PTA 乙级 1007 素数对猜想 (20分)

 1 #include<iostream>
 2 #include <cmath>
 3 using namespace std;
 4 
 5 bool isprime(int i) {
 6     for (int j = 2; j <= sqrt(i); ++j) {
 7         if (i % j == 0) {
 8             return 0;
 9         }
10     }
11     return 1;
12 }
13 int main() {
14     int n = 0;
15     int count = 0;
16     int last = -1, now = 0;
17     cin >> n;
18     for (int i = 2; i <= n; ++i) {
19         if (isprime(i)) {
20             now = i;
21             if ((now - last) == 2) {
22                 count++;
23             }
24             last = now;
25         }
26     }
27     cout << count;
28     return 0;
29 }