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

#include<cstdio>
#include<cmath>

bool isPrime(int n){
    if(n <=1 ) return  false;
    int sqr = (int)sqrt(1.0*n);
    for(int i = 2; i <= sqr; i++){
        if(n%i == 0) return false;
    }
    return true;
}
int d[111];
int main(){
    int n,radix;
    while(scanf("%d",&n) != EOF){
        if(n < 0) break;
        scanf("%d",&radix);
        if(isPrime(n) == false){
            printf("No
");
        }else{
            int len = 0;
            while(n != 0){
                d[len++] = n % radix;
                n /= radix;
            }
            for(int i = 0; i < len; i++){
                n = n *radix + d[i]; 
            }
            if(isPrime(n) == true) printf("Yes
");
            else printf("No
");
            
        }
    }
    return 0;
}

//18.8.6 
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std; 
const int maxn = 100100;
int n,radix,num[maxn];

bool isPrime(int x){
    if(x <= 1) return false;   //测试点2 
    int sqrx = sqrt(x);
    for(int i = 2; i <= sqrx; i++){
        if(x%i == 0) return false;
    }
    return true;
}

int Reverse(int x,int radix){
    int i = 0,ans = 0;
    do{
        num[i++] = x % radix;
        x /= radix;
    }while(x != 0);
    for(int j = 0; j < i; j++){
        ans = ans*radix + num[j];
    }
    return ans;
}

int main(){
    while(1){
        memset(num,0,sizeof(num));
        scanf("%d",&n);
        if(n < 0) break;
        else{
            scanf("%d",&radix);
            int u = Reverse(n,radix);
            if(isPrime(n) && isPrime(u)){
                printf("Yes
");
            }else printf("No
");
        }
        
    }
    return 0;
}