Number to Words in C++

main.cpp

#include <iostream>
#include <string>

std::string number_to_words(int num) {
  if (num == 0) {
    return "zero";
  }
  if (num < 0) {
    return "minus " + number_to_words(-num);
  }
  std::string words;
  if (num >= 1000000000) {
    words += number_to_words(num / 1000000000) + " billion ";
    num %= 1000000000;
  }
  if (num >= 1000000) {
    words += number_to_words(num / 1000000) + " million ";
    num %= 1000000;
  }
  if (num >= 1000) {
    words += number_to_words(num / 1000) + " thousand ";
    num %= 1000;
  }
  if (num >= 100) {
    words += number_to_words(num / 100) + " hundred ";
    num %= 100;
  }
  if (num >= 20) {
    static const std::string tens[] = {"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
    words += tens[num / 10] + " ";
    num %= 10;
  }
  static const std::string digits[] = {"", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};
  words += digits[num];
  return words;
}

int main() {
  std::cout << number_to_words(30) << std::endl;
  return 0;
}