1015 Reversible Primes (20 point(s))

A reversible prime in any number system is a prime whose "reverse" in that number system is also a prime. For example in the decimal system 73 is a reversible prime because its reverse 37 is also a prime.

Now given any two positive integers N (<10​5​​) and D (1<D≤10), you are supposed to tell if N is a reversible prime with radix D.

Input Specification:

The input file consists of several test cases. Each case occupies a line which contains two integers N and D. The input is finished by a negative N.

Output Specification:

For each test case, print in one line Yes if N is a reversible prime with radix D, or No if not.

Sample Input:

73 10 23 2 23 10 -2

Sample Output:

Yes Yes No

Tips:

（1）在D进制下把数字n反转：想一想是不是和在10进制下反转是一样的呢？并不用先转换为D进制，再反转，再转换为10进制。

（2）此题素数判断要求到了1e5，使用素数筛法会造成MLE。

#include<iostream> using namespace std; int reverse(int n,int d){//reverse any number with d redix int ans = 0; while(n){ ans = ans*d+n%d; n/=d; } return ans; } bool isPrime(int x){//judge if it is a prime number if(x<2) return false; for(int i=2;i*i<=x;i++){ if(x%i==0) return false; } return true; } int main(void){ int n,d; while(cin>>n){ if(n<0) break; cin>>d; if(isPrime(n)&&isPrime(reverse(n,d)))cout<<"Yes"<<endl; else cout<<"No"<<endl; } return 0; }