273. Integer to English Words

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

Example 1:

Input: 123
Output: "One Hundred Twenty Three"

Example 2:

Input: 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Example 3:

Input: 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Example 4:

Input: 1234567891
Output: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"

string numberToWords(int num) {
    vector<string> thousands{" Thousand", " Million", " Billion"};
    string res = helper(num % 1000);
    for (int i = 0; i < 3; ++i) {
        num /= 1000;
        res = (num % 1000) ? (helper(num % 1000) + thousands[i] + " " + res) : res;
    }
    while (res.back() == ' ') res.pop_back();
    return res.empty() ? "Zero" : res;
}
string helper(int num) {
    vector<string> tens{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    vector<string> ones{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    string res;
    int a = num / 100, b = num % 100, c = num % 10;
    // res = (b < 20) ? ones[b] : tens[b / 10] + (c > 0 ? " " + ones[c] : "");
    // if (a > 0) res = ones[a] + " Hundred" + (b ? " " + res : "");
    if (a > 0) res += ones[a] + " Hundred";
    if (b > 0) res += (res.empty() ? "" : " ") + ((b < 20) ? ones[b] : tens[b / 10] + (c > 0 ? " " + ones[c] : "")); 
    return res;
}