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";
        String[] units = {"", "Thousand ", "Million ", "Billion "};
        String res = "";
        for (int i = 0; num > 0; i++) {
            String cur = "";
            int tmp = num % 1000;
            if (tmp >= 100) cur += helper(tmp / 100) + "Hundred ";
            tmp %= 100;
            if (tmp >= 20) cur += helper(tmp / 10 * 10);
            tmp %= 10;
            if (tmp > 0) cur += helper(tmp);
            if (cur.length() > 0) res = cur + units[i] + res;
            num /= 1000;
        }
        res = res.trim();
        return res;
    }
    private String helper(int n) {
        if (n == 0) return "";
        String[] nums = {"", "One ", "Two ", "Three ", "Four ", "Five ",