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PAT A1015. 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


#include <stdio.h>
#include <math.h>int prime(int num) {
	int sqr, flag, i;
	if(num == 1) {
		return 0;
	}
	sqr = (int) sqrt(num);
	flag = 1;
	for(i = 2; i <= sqr; i ++) {
		if(num % i == 0) {
			flag = 0;
			break;
		}
	}
	return flag;
}
int main() {
	int num[20] = {
		-1
	};
	int N, D, i, j, reverseN;
	while(scanf("%d", &N) && N > 0) {
		scanf("%d", &D);
		if(prime(N) != 1) {
			printf("No\n");
			continue;
		}
		i = 0;
		while(N != 0) {
			num[i ++] = N % D;
			N /= D;
		}
		for(j = 0, reverseN = 0; j < i; j ++) {
			reverseN = D * reverseN + num[j];
		}
		//printf("reverseN: %d\n", reverseN);
		if(prime(reverseN) == 1) {
			printf("Yes\n");
		} else {
			printf("No\n");
		}
	}
	return 0;
}

本文标签： PAT A1015 Reversible Primes

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