numberToWords.cpp

class Solution {
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++;
        }
        trim(words);
        return words;
    }
private:
    string LESS_THAN_20[20]={"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    string TENS[10]={"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    string THOUSANDS[4]={"", "Thousand", "Million", "Billion"};
    
    string helper(int num) {
        if (num == 0)
            return "";
        else if (num < 20)
            return LESS_THAN_20[num] + " ";
        else if (num < 100)
            return TENS[num / 10] + " " + helper(num % 10);
        else
            return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
    }
    void trim(string &s) 
    {
        if (s.empty()) 
        {
            return ;
        }
        s.erase(0,s.find_first_not_of(" "));
        s.erase(s.find_last_not_of(" ") + 1);
    }
};