PAT Advanced 1015 Reversible Primes

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

将十进制数N转换为D进制数，翻转，转回十进制，判断N和其翻转是否均为素数。注意0、1在判定素数时的处理。

#include<bits/stdc++.h>
using namespace std;

int todec(string now,int radix)
{
    int len=now.length();
    int ans=0;
    for(int i=0;i<len;i++)
    {
        ans*=radix;
        ans+=now[i]-'0';
        
    }
    return ans;
}
string toarb(int now,int radix)
{
    stringstream buf;
    while(now!=0)
    {
        buf<<now%radix;
        now/=radix;
    }
    string ans;
    buf>>ans;
    reverse(ans.begin(),ans.end());
    return ans;
}
bool check(int now)
{
    if(now==0||now==1)return false;
    int up=sqrt(now);
    for(int i=2;i<=up;i++)
    {
        if(now%i==0)
            return false;
    }
    return true;
}
int main()
{
    string a,b;
    int N,D;
    
    while(cin>>N&&N>=0)
    {
        cin>>D;
    a=toarb(N,D);
    b=a;
    reverse(a.begin(),a.end());
    //cout<<a<<' '<<b<<' '<<endl;
    if(check(todec(a,D))&&check(todec(b,D)))
    {
        cout<<"Yes"<<endl;
    }else
    {
        cout<<"No"<<endl;
    }
    }
    return 0;
}