Integer to English words

Actually, straightforward to understand, but quite hard to come up with a unconfusing way of dealing with it.

If can’t come up with it properly, memorize it.

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private final String[] to19 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};

private final String[] tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

private final String[] thousands = {"", "Thousand", "Million", "Billion"};

public String numberToWords(int num) {
    if (num == 0) return "Zero";
    
    int i = 0;
    String words = "";
    
    while (num > 0) {
        if (num % 1000 != 0) words = helper(num % 1000) + thousands[i] + " " + words;
        num /= 1000;
        ++i;
    }
    return words.trim();
}

public String helper(int num) {
    if (num == 0) return "";
    else if (num < 20) return to19[num] + " ";
    else if (num < 100) return tens[num / 10] + " " + helper(num % 10);
    else return to19[num / 100] + " Hundred " + helper(num % 100);
}