My submission to: `Integer to English Words` challenge on LeetCode (Hard)

words.rb

# frozen_string_literal: true

def three_digit_to_words(num)
  ones = %w[zero one two three four five six seven eight nine]
  teens = %w[ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen]
  tens = %w[ten twenty thirty forty fifty sixty seventy eighty ninety]
  words = []

  if (num / 100).positive?
    words << ones[num / 100].capitalize
    words << 'Hundred'
    num %= 100
  end
  if (10..19).include?(num)
    i = num % 10
    words << teens[i].capitalize
    num = 0
  end
  num >= 20 ? words << tens[num / 10 - 1].capitalize : false
  (num % 10).positive? ? words << ones[num % 10].capitalize : false
  words
end

def number_to_words(num)
  words = []
  s = num.to_s.size
  if s >= 10
    words << three_digit_to_words(num / (10**9))
    (num / 10**9).positive? ? words << 'Billion' : false
    (num % 10**9).zero? ? (return words.join(' ')) : num %= 10**9
  end

  if s >= 7
    words << three_digit_to_words(num / (10**6))
    (num / 10**6).positive? ? words << 'Million' : false
    (num % 10**6).zero? ? (return words.join(' ')) : num %= 10**6
  end

  if s >= 4
    words << three_digit_to_words(num / (10**3))
    (num / 10**3).positive? ? words << 'Thousand' : false
    (num % 10**3).zero? ? (return words.join(' ')) : num %= 10**3
  end

  words << three_digit_to_words(num) if s.positive?
  num.zero? ? 'Zero' : words.flatten.join(' ')
end

# https://leetcode.com/problems/integer-to-english-words/

Link to the challenge: https://leetcode.com/problems/integer-to-english-words