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==1) return 0; 
    int flag=1;
    for(int i=2;i*i<=x;i++){
        if(x%i==0){
            flag=0;
            break;
        }
    }
    return flag;
}
int _deal(int n,int d)
{
    stack<int> s;
    while(n){
        s.push(n%d);
        n/=d;
    }
    int sum=0,t=1;
    while(!s.empty()){
        sum+=s.top()*t;
        t*=d;
        s.pop();
    }
    return sum;
}
int main()
{
    while(cin>>n&&n>0){
        cin>>d;
        int flag;
        if(!_prime(n)) flag=0;
        else{
            if(_prime(_deal(n,d))) flag=1;
            else flag=0;
        }
        if(flag) cout<<"Yes"<<endl;
        else cout<<"No"<<endl;
    }
    return 0;
}