273. Integer to English Words

Convert a non-negative integer num to its English words representation.

Example 1:

Input: num = 123
Output: "One Hundred Twenty Three"
Example 2:

Input: num = 12345
Output: "Twelve Thousand Three Hundred Forty Five"
Example 3:

Input: num = 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
Example 4:

Input: num = 1234567891
Output: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"

class Solution {
    private final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};
    public String numberToWords(int num) {
        if (num == 0)
            return "Zero";
        int i = 0;
        String words = "";
        while (num > 0) {
            if (num % 1000 != 0) 
                words = helper (num % 1000) + THOUSANDS[i] + " " + words;
            num /= 1000;
            ++i;
        }
        return words.trim();
    }
    private String helper(int num) {
            if (num == 0)
                return "";
            else if (num < 20)
                return LESS_THAN_20[num] + " ";
            else if (num < 100)
                return TENS[num / 10] + " " + helper(num % 10);
            else 
                return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
        }
}