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273. Integer to English Words 👎

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class Solution {
 public:
  string numberToWords(int num) {
    if (num == 0)
      return "Zero";
    return helper(num);
  }

 private:
  const vector<string> belowTwenty{
      "",        "One",     "Two",       "Three",    "Four",
      "Five",    "Six",     "Seven",     "Eight",    "Nine",
      "Ten",     "Eleven",  "Twelve",    "Thirteen", "Fourteen",
      "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
  const vector<string> tens{"",      "",      "Twenty",  "Thirty", "Forty",
                            "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

  string helper(int num) {
    string s;

    if (num < 20)
      s = belowTwenty.at(num);
    else if (num < 100)
      s = tens.at(num / 10) + " " + belowTwenty.at(num % 10);
    else if (num < 1000)
      s = helper(num / 100) + " Hundred " + helper(num % 100);
    else if (num < 1000000)
      s = helper(num / 1000) + " Thousand " + helper(num % 1000);
    else if (num < 1000000000)
      s = helper(num / 1000000) + " Million " + helper(num % 1000000);
    else
      s = helper(num / 1000000000) + " Billion " + helper(num % 1000000000);

    trim(s);
    return s;
  }

  void trim(string& s) {
    s.erase(0, s.find_first_not_of(' '));
    s.erase(s.find_last_not_of(' ') + 1);
  }
};

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class Solution {
  public String numberToWords(int num) {
    return num == 0 ? "Zero" : helper(num);
  }

  private final String[] belowTwenty = {"",        "One",     "Two",       "Three",    "Four",
                                        "Five",    "Six",     "Seven",     "Eight",    "Nine",
                                        "Ten",     "Eleven",  "Twelve",    "Thirteen", "Fourteen",
                                        "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
  private final String[] tens = {"",      "",      "Twenty",  "Thirty", "Forty",
                                 "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

  private String helper(int num) {
    StringBuilder s = new StringBuilder();

    if (num < 20)
      s.append(belowTwenty[num]);
    else if (num < 100)
      s.append(tens[num / 10]).append(" ").append(belowTwenty[num % 10]);
    else if (num < 1000)
      s.append(helper(num / 100)).append(" Hundred ").append(helper(num % 100));
    else if (num < 1000000)
      s.append(helper(num / 1000)).append(" Thousand ").append(helper(num % 1000));
    else if (num < 1000000000)
      s.append(helper(num / 1000000)).append(" Million ").append(helper(num % 1000000));
    else
      s.append(helper(num / 1000000000)).append(" Billion ").append(helper(num % 1000000000));

    return s.toString().trim();
  }
}

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class Solution:
  def numberToWords(self, num: int) -> str:
    if num == 0:
      return 'Zero'

    belowTwenty = ['',        'One',       'Two',      'Three',
                   'Four',    'Five',      'Six',      'Seven',
                   'Eight',   'Nine',      'Ten',      'Eleven',
                   'Twelve',  'Thirteen',  'Fourteen', 'Fifteen',
                   'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen']
    tens = ['',      'Ten',   'Twenty',  'Thirty', 'Forty',
            'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety']

    def helper(num: int) -> str:
      if num < 20:
        s = belowTwenty[num]
      elif num < 100:
        s = tens[num // 10] + ' ' + belowTwenty[num % 10]
      elif num < 1000:
        s = helper(num // 100) + ' Hundred ' + helper(num % 100)
      elif num < 1000000:
        s = helper(num // 1000) + ' Thousand ' + helper(num % 1000)
      elif num < 1000000000:
        s = helper(num // 1000000) + ' Million ' + helper(num % 1000000)
      else:
        s = helper(num // 1000000000) + ' Billion ' + helper(num % 1000000000)
      return s.strip()

    return helper(num)