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).

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

正确做法

#include <iostream>
#include <cmath>

using namespace std;

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

int main() {
    int x;
    cin >> x;
    if ((isPrime(x) && isPrime(x - 6))) {
        printf("Yes\n");
        printf("%d", x - 6);
    } else if (isPrime(x) && isPrime(x + 6)) {
        printf("Yes\n");
        printf("%d", x + 6);
    } else {
        printf("No\n");
        while (1) {
            x++;
            if (isPrime(x) && (isPrime(x + 6) || isPrime(x - 6))) break;
        }
        printf("%d", x);
    }
    return 0;
}

缺少判断,在while循环的时候

#include <cstring>
#include <iostream>

using namespace std;
const int N = 110;
bool res = true;

bool is_prime(int x)
{
    for(int i = 2; i <= x / i; i++)
    {
        if (x % i == 0) return false;
    }
    return true;
}
int check(int m)
{   
    if (is_prime(m))
    {
        bool f1 = is_prime(m - 6);
        bool f2= is_prime(m + 6);
        if (f1) return m - 6;
        else if (f2) return m + 6;
        else{
            m++;
            res = false;
            while(true)
            {
                bool f1 = is_prime(m);
                bool f2 = is_prime(m - 6);
                bool f3 = is_prime(m + 6);
                if (f1 && f2) return m;
                else if (f1 && f2) return m;
                m++;
            }
        }
    }else{
        m++;
        res = false;
        while(true)
        {
            bool f1 = is_prime(m);
            bool f2 = is_prime(m - 6);
            bool f3 = is_prime(m + 6);
            if (f1 && f2) return m;
            else if (f1 && f2) return m;
            m++;
        }
    }
}

int main()
{
    int m;
    cin >> m;
    int k = check(m);
    if (res) puts("Yes");
    else puts("No");
    printf("%d", k);
}