Given a nonnegative integer, num, convert it to its English word representation.

Constraints:

• $0 \le$ num $\le 2^{31}-1$

To solve this problem, we need to understand the different types of words that can appear in the word representation of a number, which depend on three factors:

1. Digits: There are ten digits ranging from 0 to 9.

2. Number of digits: In our case, the maximum number of digits could be $10$ because the input number, num, will be in the range [0, 2147483647].

3. Positional values: The positional value, also known as place value, of a digit can be ones (units), tens, hundreds, thousands, ten thousands, hundred thousands, millions, ten millions, hundred millions, or billions.

Based on these factors, numbers in different ranges have specific word representations. Here’s a general workflow for converting numbers to their word representation:

1. Numbers from $0$ to $19$: The numbers in the range $[0, 19]$ have unique word representations.

2. Numbers from $20$ to ...