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 10^810
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
【声明】
由于此题还未上PAT官网题库，故没有测试集，仅仅是通过了样例，若发现错误，感谢留言指正。
Solution：
这道题很简单，就是判断一下n 以及 n-6 或者 n+6是素数，不是的话，那就n++，直到满足要求
1 #include <iostream> 2 using namespace std; 3 bool isPrime(int x)//判断是不是素数 4 { 5 if (x < 3) 6 return x >= 1; 7 for (int i = 2; i*i <= x; ++i) 8 if (x%i == 0) 9 return false; 10 return true; 11 } 12 int isSexyPrimes(int x)//判断满不满足题目要求 13 { 14 if (isPrime(x)) 15 { 16 bool fL = isPrime(x - 6), fR = isPrime(x + 6); 17 if (fL || fR) 18 return fL ? x - 6 : x + 6; 19 } 20 return -1; 21 } 22 int main() 23 { 24 int n; 25 cin >> n; 26 int res = isSexyPrimes(n); 27 if (res > 0) 28 { 29 printf("Yes "); 30 printf("%d ", res); 31 } 32 else 33 { 34 printf("No "); 35 for (int i = n + 1; i < INT32_MAX; ++i)//另寻找大数，直至满足要求 36 { 37 if (isSexyPrimes(i) > 0) 38 { 39 printf("%d ", i); 40 break; 41 } 42 } 43 } 44 return 0; 45 }