Solution.java

class Solution {
    private static final String[] LESS_THAN_20 = {
        "Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",
        "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"
    };

    private static final String[] TENS = {
        "", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"
    };

    private static final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};

    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }

        String words = "";
        for (int i = 0; i < THOUSANDS.length; i++) {
            if (num % 1000 != 0) {
                words = helper(num % 1000) + THOUSANDS[i] + " " + words;
            }
            num /= 1000;
        }

        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);
        }
    }
}