【PTA Advanced】1156 Sexy Primes（C++）

目录

题目

Input Specification:

Output Specification:

Sample Input 1:

Sample Output 1:

Sample Input 2:

Sample Output 2:

思路

代码

---

题目

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 (≤108).

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

思路

这题简单到离谱的地步！

唯一需要注意的点是测试点2：素数应该是一个正数，所以在判断n-6是否是素数之前要先判断一下n-6是否是正数。

代码

#include <iostream>
#include <cmath>using namespace std;// 判断x是否为素数
bool isPrime(int x) {if(x==1) return false;for(int i=2;i<=sqrt(x);i++)if(x%i==0) return false;return true;
} int main(int argc, char** argv) {int n;cin>>n;if(isPrime(n)) {if(n-6>0&&isPrime(n-6)) {printf("Yes\n%d",n-6);return 0;}else if(isPrime(n+6)) {printf("Yes\n%d",n+6);return 0;}}cout<<"No"<<endl;while(true) {n++;if(isPrime(n)&&(n-6>0&&isPrime(n-6)||isPrime(n+6))) {cout<<n;break;}}return 0;
}