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

题目大意：给两个数N,d。判断N用d进制表示的反转数是否为质数，以及N本身是不是质数； 总的来说就是判断质数，进制转换
注意输入结束不是一-2为标志，而是以负数为标志。之前一直用-2作为结束的标志，浪费了不少的时间找bug

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

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

int reverse(int num, int d){
  int ans = 0, temp;
  while(num){
    temp = num%d;
    ans = ans*d + temp;
    num /= d;
  }
  return ans;
}
int main(){
  int num, d;
  while(true){
    scanf("%d", &num);
    if(num<0) break;
    scanf("%d", &d);
    if(is_prime(num) && is_prime(reverse(num, d))) printf("Yes
");
    else printf("No
");
  }
  return 0;
}