Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

For example,

123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

class Solution {
private:
    string helper(int num) {
        const string words20[] = { "One", "Two", "Three", "Four", "Five", "Six",
                "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen",
                "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen",
                "Nineteen" };
        const string words100[] = { "Twenty", "Thirty", "Forty", "Fifty",
                "Sixty", "Seventy", "Eighty", "Ninety" };

        if (num >= 1000000000)
            return helper(num / 1000000000) + " Billion" + helper(num % 1000000000);
        else if (num >= 1000000)
            return helper(num / 1000000) + " Million" + helper(num % 1000000);
        else if (num >= 1000)
            return helper(num / 1000) + " Thousand" + helper(num % 1000);
        else if (num >= 100)
            return helper(num / 100) + " Hundred" + helper(num % 100);
        else if (num >= 20)
            return " " +words100[num / 10 - 2] + helper(num % 10);
        else if (num >= 1)
            return " "+ words20[num - 1];
        else
            return "";
    }
public:
    string numberToWords(int num) {
        if (num == 0)
            return "Zero";
        else
            return helper(num).substr(1);
    }
};