PAT 1015. Reversible Primes

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <vector>



using namespace std;



void init_primes(vector<char>& primes) {

    int len = primes.size();

    for (int i=0; i<len && i<2; i++) primes[i] = false;

    if (len < 3) return;

    primes[2] = true;

    

    for (int i=2; i<len; i++) {

        if (!primes[i]) continue;

        int v = i * 2;

        while (v < len) {

            primes[v] = false;

            v += i;

        }

    }

    

}



int inv_value(int n, int radix) {

    int base = 1;

    int v = 0;

    int r = 0;

    vector<int> rs;

    while (n) {

        r = n % radix;

        n = n / radix;

        rs.push_back(r);

    }

    for (int i=rs.size() - 1; i>=0; i--) {

        v += rs[i] * base;

        base *= radix;

    }

    return v;

}



int main() {

    const int MAX_N = 100000;

    vector<char> primes(MAX_N + 1, true);

    init_primes(primes);

    

    int num, radix;



    for (;;) {

        scanf("%d", &num);

        if (num < 0) break;

        scanf("%d", &radix);

        

        if (!primes[num]) {

            printf("No\n");

            continue;

        }

        int iv = inv_value(num, radix);

        if (!primes[iv]) {

            printf("No\n");

        } else {

            printf("Yes\n");

        }

    }

    

    return 0;

}

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