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

题意：

　　给出一个十进制数字N，将这个数字用D进制表示并反转后，问这个数字还是不是素数。

思路：

　　字符串 + 模拟。

Code：

#include <bits/stdc++.h>

using namespace std;

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

int main() {
    int n, d;
    cin >> n;
    while (n > 0) {
        cin >> d;
        string temp = "";
        int r = n;
        while (r > 0) {
            temp += to_string(r % d);
            r /= d;
        }
        int index = 0;
        for (int i = temp.size() - 1; i >= 0; --i)
            r += (temp[i] - '0') * pow(d, index++);
        if (isPrime(n) && isPrime(r)) {
            cout << "Yes" << endl;
        } else {
            cout << "No" << endl;
        }
        cin >> n;
    }

    return 0;
}