甲级PAT 2019年3月 Sexy Primes （20 分)

7-1 Sexy Primes （20 分)

Sexy primes are pairs of primes of the form (p, p+6), so-named since "sex" is the Latin word for "six". (Quoted from http://mathworld.wolfram.com/SexyPrimes.html)

Now given an integer, you are supposed to tell if it is a sexy prime.

Input Specification:

Each input file contains one test case. Each case gives a positive integer N (≤10​8​​).

Output Specification:

For each case, print in a line Yes if N is a sexy prime, then print in the next line the other sexy prime paired with N (if the answer is not unique, output the smaller number). Or if N is not a sexy prime, print No instead, then print in the next line the smallest sexy prime which is larger than N.

Sample Input 1:

47

Sample Output 1:

Yes
41

Sample Input 2:

21

Sample Output 2:

No
23

完整代码 

#include
using namespace std;

bool isprime(int x){
	if(x<=1) return false;
	for(int i=2;i*i<=x;i++){
		if(x%i==0) return false;
	} 
	return true;
} 

bool issix(int n){
	if(isprime(n)){
		if(isprime(n-6)){
			return true;
		}
		if(isprime(n+6)){
			return true;
		}
	}
	return false;
}

int main(){
	int n,i;
	cin>>n;
	if(isprime(n)){
		if(isprime(n-6)){
			cout<<"Yes"<