[LeetCode] Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 2³¹ - 1.
For example,

123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

There are several key observations to solve this:
1. the number can be divided into numbers of 3 digits long;
2. for every 3 digits, words like ""Thousand", "Millions", "Billion" are added
3. if the last two digits of the 3 digits form a number falls between 10 to 19, we treat them together; otherwise we treat them separately.
4. read the digits from left to right until we process all digits

Here is the source code: