【PAT】1015. Reversible Primes (20)

题目链接：http://pat.zju.edu.cn/contests/pat-a-practise/1015

题目描述：

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 (< 10⁵) 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：给出一个数N和一个基数D。首先这个数必须是素数。其次，将这个数（1）转为D进制数（2）将这个D进制数反转（3）将反转后的数再转为十进制数，这个十进制数依然是素数。 这样我们就输出“Yes”。

题目不难，就是表述不清楚，看的费劲。

判断素数：一个数N，如果从2到sqrt(N)都不存在因子，则认为这个数是素数。 注意：0和1不是素数。

将十进制数转为n进制数：（待补充。。。）

#include<iostream>
#include<string.h>
#include<math.h>
using namespace std;

bool is_Prime(int a)
{
	int i;
	if(a==0 || a==1) return false;

	//注意是i<=sqrt(),之前弄成i<sqrt,一直出错
	for(i=2; i <= sqrt((double)a); i++){
		if( a%i == 0) return false;
	}

	return true;
}

int change(int n,int d){
	int a[100000];
	memset(a,0,sizeof(a));
	int total = 0;
	int j;
	int i;
	for(i=0; ; i++){
		a[i] = n%d;
		n /= d;
		if(n==0) break;
	}

	for(j=0; j<=i; j++)
	{
		total = total*d + a[j];
	}
/*也可以这样写
	do 
	{
		total = total*d + n%d;
		n/=d;
	} while (n != 0);
*/
	return total;
}

int main()
{
	int N,D;
	while(cin>>N){
		if( N<0 ) break;
		cin>>D;
	    if( is_Prime(N) && is_Prime( change(N,D) )){
			cout<<"Yes"<<endl;
		}
		else{
			cout<<"No"<<endl;
		}
	}
	return 0;
}