PAT 甲级 1015 Reversible Primes（20）

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 (<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

解题流程：

对于每行输入的两个数据N和D

将N转化为D进制的数（字符串），各位数字（字符）倒叙

倒叙后按进制D还原为10进制，得到数字rN

判断条件：如果rN和N都为素数，输出Yes，否则输出No

结束条件：N为负数

#include<iostream>
#include<string>
#include<cmath>
using namespace std;
int judge(int a) {      //判断素数
	if (a == 1)
		return 0;
	for (int i = 2; i <= sqrt(a); i++)
		if (a%i == 0)
			return 0;
	return 1;
}
int trans(int N, int D) {      //数字转化为D进制--逆转--还原10进制
	string str;
	for (; N != 0; N /= D)
		str += to_string(N%D);
	return stoi(str, 0, D);
}
int main() {
	int N, D;
	while (1) {
		cin >> N >> D;     //N 、D可分开判断N
		if (N < 0)	break;
		if (judge(N) && judge(trans(N, D)))
			cout << "Yes" << endl;
		else
			cout << "No" << endl;
	}
	return 0;
}