1015 Reversible Primes (20分) 甲级真题

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

分析：判断输入的数字是否大于0，若大于0则判断是否为素数，若是，则将其转换成d进制，然后从十进制判断是否为素数。是就输出Yes，否则输出No。

#include<iostream>
#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 num, code, sum;
    for (int i = 0; true; i++) {
        sum = 0;
        cin >> num;
        if (num > 0) {
            cin >> code;
            if (!isprime(num)) { cout << "No\n"; continue; }
            while (num) {
                sum = sum * code + num % code;
                num /= code;
            }
            if (isprime(sum)) cout << "Yes\n";
            else cout << "No\n";
        }
        else break;
    }
    return 0;
}