PAT 甲级 1015 Reversible Primes (20)（20 分）

1015 Reversible Primes (20)（20 分）

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

# include <iostream>
# include <cstdio>
# include <cmath>
using namespace std;

bool isprime(int N){
	if(N <= 1)
		return false;
	for(int i = 2; i <= sqrt(N); i++){
		if(N % i == 0){
			return false;
		}
	}
	return true;
}

int main(){
//	freopen("C:\\1.txt", "r", stdin); 
	int N, D;
	while(scanf("%d", &N) != EOF){
		if(N < 0) break;
		scanf("%d", &D);
		long long jz = 0;
		int a[100], len = 0;
		if(!isprime(N)) {
			printf("No\n");
			continue;
		}
		
		for(int i = 0; N; i++, len++){
			a[i] = N % D;
			N /= D;
		}
		for(int i = 0; i < len; i++){
			jz = jz * D + a[i];
		}
		if(!isprime(jz))
			printf("No\n");
		else
			printf("Yes\n");
	}
	return 0;
}