PAT 1015. Reversible Primes

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 1023 223 10-2

Sample Output:

YesYesNo

分析

先判断数是否是prime，然后化为radix进制，再反转，再化成十进制判断是不是prime；看了半天才明白什么意思

代码如下

#include
    
     #include
     
      using namespace std;long long int toradix(long long n,long long int radix){    string num;    while(n!=0){        num.insert(num.begin(),1,'0'+n%radix);        n/=radix;    }    return stoll(num);}bool isprime(long long int num){    if(num<=1) return false;    for(int i=2;i<=sqrt(num);i++)    if(num%i==0) return false;    return true;}long long int toten(string n,long long int radix){    long long int m=0,tag=0;    for(int i=n.size()-1;i>=0;i--)    m+=pow(radix,tag++)*(n[i]-'0');    return m;}int main(){    long long int radix,num,n;    while(1){    int flag=1; string m,s;    cin>>n;    if(n<0) return 0;    cin>>radix;    if(!isprime(n)) flag=0;     n=toradix(n,radix);     s=to_string(n);    for(int i=s.size()-1;i>=0;i--)    m.append(1,s[i]);     n=toten(m,radix);    if(!isprime(n)) flag=0;    flag>0?cout<<"Yes"<
      
       <<"No"<