PAT甲级1015

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

#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;
bool isPrime(int a)
{
  if (a < 2)
    return false;
  for (int i = 2; i <= sqrt(a); i++)
  {
    if (a % i == 0)
    {
      return false;
    }
  }
  return true;
}
bool reversePrime(int a, int radix)
{
  if (isPrime(a))
  {
    vector<int> v;
    do
    {
      v.push_back(a%radix);
      a /= radix;
    } while (a);
    int sum = 0;
    for (int i = 0; i < v.size(); i++)
    {
      sum += v[i] * pow(radix, v.size()-1-i);
    }
    if (isPrime(sum))
      return true;
    else
      return false;
  }
  else
    return false;
}
int main()
{
  int N, radix;
  while (1)
  {
    scanf("%d", &N);
    if (N < 0)
      break;
    scanf("%d", &radix);
    if (reversePrime(N, radix))
    {
      printf("Yes\n");
    }
    else
      printf("No\n");
  }
  return 0;
}