273. Integer to English Words

T: O(n), the total only loop entire number length n

S: O(1), only ouput String and static array

class Solution {
    private static String[] belowTen = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    private static String[] belowTwenty = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private static String[] belowHundred = {"", "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) {
        StringBuilder result = new StringBuilder();
        
        if (num < 10) {
            result.append(belowTen[num]);
        } else if (num < 20) {
            result.append(belowTwenty[num - 10]);
            
        } else if (num < 100) {
            result.append(belowHundred[num/10]).append(" ").append(helper(num%10));
            
        } else if (num < 1000) {
            result.append(helper(num/100)).append(" Hundred ").append(helper(num%100));
            
        } else if (num < 1000000) {
            result.append(helper(num/1000)).append(" Thousand ").append(helper(num%1000));
            
        } else if (num < 1000000000) {
            result.append(helper(num/1000000)).append(" Million ").append(helper(num%1000000));

        } else {
            result.append(helper(num/1000000000)).append(" Billion ").append(helper(num%1000000000));
        }
        return result.toString().trim();
    }
}
/*
Input: num = 1,234,567
Output: "One [Million] Two Hundred Thirty Four [Thousand] Five Hundred Sixty Seven"


1,234,567

1,000,010


1,234,234,234
One [Billion] Two Hundred Thirty Four [Million] Two Hundred Thirty Four [Thousand] Two Hundred Thirty Four

1,099,099,099
One [Billion] "" Ninety nine [Million] Ninety nine [Thousand] Ninety nine

1,000,000,000
One [Billion] => zero is ""
*/