1015 Reversible Primes (20)（20 point(s)）

problem

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 (< 10^5^) 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

answer

#include<bits/stdc++.h>
using namespace std;

int prime[100000] = {2, 3};
set<int> s;

void Prime(){
    s.insert(2);
    s.insert(3);
    int i, j, flag, ta = 2;
    for(i = 5; i < 100010; i+=2){
        for(j = 0, flag = 1; prime[j]*prime[j] <= i; j ++){
            if(i % prime[j] == 0) {
                flag = 0; break;
            }
        }
        if(flag){
            prime[ta++] = i;
            s.insert(i);
        }
    }
}

bool isPrime(int num)
{
    set<int>::iterator it;
    it = s.find(num);
    if(it != s.end()) return true;
    else return false;
}

int Reverse(int a, int d){
    vector<int > s;
    while(a > 0){
        s.push_back(a%d);
        a/=d;
    }
    int num = 0;
    reverse(s.begin(), s.end());
    for(int i =0;  i< s.size(); i++){
        num += s[i]*pow((float)d, i);
//      cout<<s[i];
    }
    return num;
}
int main(){
    ios::sync_with_stdio(false);
//  freopen("test.txt", "r", stdin);
    
    Prime();
    int N, M;
    while (cin>>N){
        if(N < 0)  break;
        cin>>M;
        
        if(!isPrime(N) || !isPrime(Reverse(N,M))) {
            cout<<"No"<<endl;
        }else{
            cout<<"Yes"<<endl;
        }
    }
    return 0;
}

experience

• 素數曬法，背下模板。