[PAT-甲级]1015.Reversible Primes

1015. Reversible Primes (20)

时间限制

400 ms

内存限制

65536 kB

代码长度限制

16000 B

判题程序

Standard

作者

CHEN, Yue

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

解题思路：输入一个数n和d。判断n是不是素数，以及n在d进制下，用d进制表示n这个数的反转过来，再转化成十进制，看这个数是不是素数，如果都是素数则打印YES，否则打印No.

#include<iostream>
using namespace std;

bool isPrime(int num);
int reverseNumber(int num, int radix);

int main()
{
int n, d;
while(cin>>n)
{
if(n < 0)
break;
cin>>d;
int rev_n = reverseNumber(n, d);
if(isPrime(n) && isPrime(rev_n))
cout<<"Yes"<<endl;
else
cout<<"No"<<endl;
}
return 0;
}

bool isPrime(int num)
{
if(num < 2)
return false;
if(num ==2 || num == 3)
return true;
for(int i = 2; i*i <= num; i ++)
if(num % i == 0)
return false;
return true;
}

int reverseNumber(int num, int radix)
{
int rev_num = 0;
while(num != 0)
{
rev_num *= radix;
rev_num += num % radix;
num /= radix;
}
return rev_num;
}