PAT 2019-3 7-1 Sexy Primes

Description:

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 (≤).

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

Keys:

• 素数

Attention:

• 这几次很爱考素数，奶一口欧拉算法，9月份考试能不能中

Code:

/*
SP数，P和P+6都是素数
给定一个正整数，判断N是否为SP数，打印与他配对的另一个素数（如果不唯一，输出较小的一个
如果N不是SP数，打印大于N的最小SP数

1，判断i-6是否大于n
2，n-6可能小于0,
*/
#include<cstdio>

bool IsPrime(long long n)
{
    if(n <= 1)
        return false;
    for(long long i=2; i*i<=n; i++)
        if(n%i == 0)
            return false;
    return true;
}

int main()
{
#ifdef ONLINE_JUDGE
#else
    freopen("Test.txt", "r", stdin);
#endif // ONLINE_JUDEG

    int n;
    scanf("%d",&n);
    if(IsPrime(n-6) && IsPrime(n))
        printf("Yes\n%d", n-6);
    else if(IsPrime(n) && IsPrime(n+6))
        printf("Yes\n%d", n+6);
    else
    {
        for(int i=n+1; i<1e9; i++)
            if(IsPrime(i-6) && IsPrime(i))
            {
                printf("No\n%d", i-6>n?i-6:i);
                break;
            }
    }

    return 0;
}