Find maximum possible time HH:MM by permuting four given digits

Question

I recently took a coding test for a promotion at work. This was one of the tasks I really struggled with and was wondering what is the best way to do this. I used a load of

Answer 1

I recently was working on the same problem (with 6 digits though) and came up with this non-brute solution:

  #include 
  #include 

  int numbers[6] = { 0, 0, 0, 0, 0, 0 };
  int input[6] = { 1, 7, 3, 3, 4, 1 };

  void buildHistogram() {
      for (int i = 0; i < 6; ++i) {
          numbers[input[i]]++;
      }
  }

  int getMaxNotExceeding(int number) {
      for (int i = number; i >= 0; --i) {
          if (numbers[i] > 0) {
              numbers[i]--;
              return i;
          }
      }
      throw std::exception("CANNOT CREATE TIME");
  }

  int main() {
      try {
          buildHistogram();
          int hours = (getMaxNotExceeding(2) * 10);
          if (hours < 20) {
            hours += getMaxNotExceeding(9);
          } else {
            hours += getMaxNotExceeding(3);
          }
          int minutes = (getMaxNotExceeding(5) * 10) + getMaxNotExceeding(9);
          int seconds = (getMaxNotExceeding(5) * 10) + getMaxNotExceeding(9);

          if (seconds > 59 || minutes > 59 || hours > 23) {
              throw std::exception("CANNOT CREATE TIME");
          }
          std::cout.fill('0');
          std::cout << std::setw(2) << hours << ':' << std::setw(2) << minutes << ':' << std::setw(2) << seconds << std::endl;
      } catch(const std::exception& ex) {
          std::cout << ex.what() << std::endl;
      }
      return 0;
  }