LeetCode 273 Integer to English

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 2^31 - 1.

Hint:

• Group the digits by thousand using mod operator
• Use a helper function to convert the chunk to words

Edge case:

• Input is zero

class Solution {
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        String[] ones = {"One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen", "Twenty"};
        String[] tens = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety", "Hundred"};
        String[] thousands = {" ", "Thousand", "Million", "Billion"};
        String ret = "";
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                ret = format(num % 1000, ones, tens) + thousands[i] + " " + ret;            
            }
            num = num / 1000;
            i ++;
        }
        return ret.trim();
    }
    private String format(int num, String[] ones, String[] tens) {
        StringBuilder sb = new StringBuilder();
        if (num >= 100) {
            sb.append(ones[num / 100 - 1]).append(" ").append("Hundred ");
            num = num % 100;
        }
        if (num > 20) {
            sb.append(tens[num / 10 - 2]).append(" ");
            num = num % 10;
        }
        if (num > 0) {
            sb.append(ones[num - 1]).append(" ");            
        }
        return sb.toString();
    }
}