PAT_A　1015. Reversible Primes (20)

1015. Reversible Primes (20)

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：反转素数是指一个素数反转后仍是一个素数。
• CODE:
  
  • 判断素数算法
  • 反转

#include <iostream>
#include<cmath>
using namespace std;
/** reversible num base on radix d* test the two num which radix is 10 is prime*/
long reverse(long a,int b)
{long c=0;long out=0;long count=0;do{count++;c*=10;c+=a%b;a/=b;}while(a>=1);//形成　radix:b形式的数c，方便检查错误long tmp=0;//将c转化成十进制数，再进行判断素数for(long i=0;i<count;i++){tmp=c%10;//一开始写成了　c%b　最后两个测试点出错！细节呀！c/=10;out+=pow(b,i)*tmp;}return out;
}
//这个反转不错，上边的反转形成了中间数，这个更简洁
long  reverse2(long n,int radix)  
{  long num=n%radix;   while((n/radix)!=0)  {  n/=radix;                 num = num*radix + n%radix;                }  return num;  
} 
//只判断前开方
bool isPrime1(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++){  //写成　i*i<a也行if( a%i == 0) return false;  }  return true;  
}  
//只判断前一半
bool isPrime(int a)
{//别忘了１　、　２if(a==1)return false;if(a==2)return true;for(int i=2;i<a/2+1;i++){if(a%i==0)return false;}return true;
}
bool isPrime(int a,int b)
{//自身及反转是否是素数if(isPrime(a)==false)return false;elsereturn isPrime(reverse(a,b));
}
int main()
{int data=10;int radix=10;while(true){cin>>data;if(data<0)return 0;cin>>radix;if(isPrime(data,radix)==false)cout<<"No"<<endl;elsecout<<"Yes"<<endl;}return 0;
}

• 参考
  
  • 浙江大学PAT上机题解析之1015. Reversible Primes (20)
  • 【PAT】1015. Reversible Primes (20)