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 (
) and D (
), 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
解析
体会到了甲级和乙级的区别了。同样是水题,但是甲级会让你做更多的事情。也就是一道题考了几个点。
什么是Reversible Primes?
这个数是素数的同时,将其转换为D进制的数,再反转,产出的数还是个素数。就是Reversible Primes
又考了素数,又考了进制转换 d=====( ̄▽ ̄*)b
Code:
#include<iostream>
#include<algorithm>
#include<string>
using namespace std;
bool isprime(unsigned num) {
if (num <= 1)
return false;
for (int i = 2; i*i <= num; i++)
if (num%i == 0)
return false;
return true;
}
unsigned rever(int num, int radix) {
string result;
do {
result += num % radix+'0';
num /= radix;
} while (num);
int sum = 0;
for (auto x : result) {
sum = sum * radix + x - '0';
}
return sum;
}
int main()
{
int N, D;
while ((cin >> N) && N >= 0) {
cin >> D;
if (isprime(N) && isprime(rever(N, D)))
cout << "Yes" << endl;
else
cout << "No" << endl;
}
}