[LeetCode] Integer to English Words

Problem

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

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"

Solution

class Solution {
    private String[] belowTen = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    private String[] tenToTwenty = new String[] {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private String[] tens = new String[] {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        return helper(num);
    }
    
    private String helper(int num) {
        String result;
        if (num < 10) result = belowTen[num];
        else if (num < 20) result = tenToTwenty[num-10];
        else if (num < 100) result = tens[num/10] + " " + belowTen[num%10];
        else if (num < 1000) result = belowTen[num/100] + " Hundred " + helper(num%100);
        else if (num < 10000) result = belowTen[num/1000] + " Thousand " + helper(num%1000);
        else if (num < 1000000) result = helper(num/1000) + " Thousand " + helper(num%1000);
        else if (num < 1000000000) result = helper(num/1000000) + " Million " + helper(num%1000000);
        else result = helper(num/1000000000) + " Billion " + helper(num%1000000000);
        return result.trim();
    }
}