递归算法求进制转换

#include <iostream>
#include <sstream> 

using namespace std;
int r, g, b;

void helper(int n, ostringstream& out) {
  if (!n) return;
  helper(n / 13, out);
  if (n % 13 > 9) out << string(1, n % 13 - 10 + 'A');
  else out << string(1, n % 13 + '0');
}

string conv(int n) {
  if (!n) return "00";
  ostringstream out;
  helper(n, out);
  string ans = out.str();
  return ans.length() < 2 ? '0' + ans : ans;
}

int main(void) {
  cin >> r >> g >> b;
  printf("#%s%s%s", conv(r).c_str(), conv(g).c_str(), conv(b).c_str());
  return 0;
}