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"

public class Solution {
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        
        String res = "";
        int count = 0;
        while (num > 0) {
            int digit = num % 1000;
            num /= 1000;
            
            String tmp = helper(digit);
            res = tmp + " " + units[count] + " " + res;
            count++;
        }
        
        return res.trim();
    }
    
    private String helper(int num) {
        if (num == 0) return "";
        
        String res = "";
        if (num / 100 > 0) {
            res = digits[num / 100] + " Hundred ";
            num %= 100;
        }
        
        if (num / 10 > 1) {
            res += tens[num / 10] + " ";
            num %= 10;
        } else if (num / 10 == 1) {
            res += teens[num % 10] + " ";