Math — Integer to English Words

solve(num) — convert a non-negative integer (0 <= num < 2^31) to its English words. Use
the exact format below so the result is unique: words are Capitalised, separated by single
spaces, no "and" and no hyphens.

• 0 is "Zero";
• tens above nineteen are Twenty, Thirty, Forty, Fifty, Sixty, Seventy, Eighty, Ninety;
• groups of three digits are followed by Thousand, Million or Billion, and a group of

000 is skipped entirely.

Split the number into groups of three digits from the right. Write a helper for a number
below 1000 (hundreds, then the tens / teens / ones), then glue the groups together with their
scale word, from the largest to the smallest.

Examples

solve(123)       → "One Hundred Twenty Three"
solve(12345)     → "Twelve Thousand Three Hundred Forty Five"
solve(1000000)   → "One Million"
solve(0)         → "Zero"