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"

class Solution {
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        StringBuilder sb = new StringBuilder();
        String[] one = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        String[] ten = {"Zero", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String[] units = {"Zero", "Thousand", "Million", "Billion", "Trillion"};
        
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                sb.insert(0, units[i] + " ");
                numToWordsHelper(sb, one, ten, num % 1000);