PAT (Advanced Level) Practice 1015 Reversible Primes (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 (<) and D (1), 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

思路：先判断自身是否是素数，再转换为对应进制的数，再判断是否是素数。

 #include <iostream>
 #include <string.h>
 #include <cstring>
 #include <algorithm>
 #include <cstdio>
 #include <stack>
 using namespace std;
 int n,d;
 int _prime(int x)
 {
     if(x==) return ;
     int flag=;
     for(int i=;i*i<=x;i++){
         if(x%i==){
             flag=;
             break;
         }
     }
     return flag;
 }
 int _deal(int n,int d)
 {
     stack<int> s;
     while(n){
         s.push(n%d);
         n/=d;
     }
     int sum=,t=;
     while(!s.empty()){
         sum+=s.top()*t;
         t*=d;
         s.pop();
     }
     return sum;
 }
 int main()
 {
     while(cin>>n&&n>){
         cin>>d;
         int flag;
         if(!_prime(n)) flag=;
         else{
             if(_prime(_deal(n,d))) flag=;
             else flag=;
         }
         if(flag) cout<<"Yes"<<endl;
         else cout<<"No"<<endl;
     }
     return ;
 }