PAT 甲级 1015. Reversible Primes (20)

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 (< 105) 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<string>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<queue>
#include <cmath>
using namespace std;

bool isprime(int n) {
if (n <= 1) return false;
int sqr = int(sqrt(n*1.0));
for (int i = 2; i <= sqr; i++) {
if (n%i == 0)
return false;
}
return true;
}

int main() {
int n, d;
while (scanf("%d", &n) != EOF) {
if (n < 0) break;
scanf("%d", &d);
if (isprime(n) == false) {
printf("No\n");
continue;
}
int len = 0;
int arr[100];
do {
arr[len++] = n%d;
n = n / d;
} while (n != 0);
for (int i = 0; i < len; i++) {
n = n*d + arr[i];
}
if (isprime(n) == false) {
printf("No\n");
}
else {
printf("Yes\n");
}
}
return 0;
}