5.4 PAT A1015 Reversible Primes (20分)(D进制反转素数）

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

题意为判断将素数转换为d进制后反转是否还是素数 。注意1要特判，否则会有一组错误。

参考代码：

#include<cstdio>
#include <cmath>

bool isprime(int n)
{
	if(n<=1) //注意n==1时要特判为返回false 
	return false;
	int sqr=(int)sqrt(1.0*n);
	for(int i=2;i<=sqr;++i)
	{
		if(n%i==0)
			return false;
	}
	return true;
}
int change(int n,int a)
{
	int b,ans=0;
	do{
		b=n%a;
		n/=a;
		ans=ans*a+b; //反转+转回十进制 
	}while(n);
	return ans;
}
int main()
{
	int n,d;
	while(scanf("%d",&n)&&n>=0)
	{
		scanf("%d",&d);
		int b=change(n,d);
		if(isprime(n)&&isprime(b))
			printf("Yes\n");
		else
			printf("No\n");
	}
	return 0;
}