PAT 甲级 1156 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

【AC代码】简单素数判断—不能再简单了~

#include<bits/stdc++.h>
using namespace std;

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

int main()
{
    int n; cin >> n;
    if(isPrime(n))
    {
        if(isPrime(n-6)) 
        {
            cout << "Yes" << endl << n-6;
            return 0;
        }
        else if(isPrime(n+6))
        {
            cout << "Yes" << endl << n+6;
            return 0;
        }
    }
     else{
         while(1)
         {
             n+=1;
             if(isPrime(n) && isPrime(n+6))
             {
                 cout << "No" << endl << n; return 0;    
            }
            if(isPrime(n) && isPrime(n-6))
             {
                 cout << "No" << endl << n; return 0;    
            }
        }
     }
 }