[CareerCup] 17.7 English Phrase Describe Integer 英文单词表示数字

string convert_hundred(int num) {
    vector<string> v1{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    vector<string> v2{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eigthy", "Ninety"};
    string res;
    int a = num / 100, b = num % 100, c = num % 10;
    res = b < 20 ? v1[b] : v2[b / 10] + (c ? " " + v1[c] : "");
    if (a > 0) res = v1[a] + " Hundred" + (b ? " " + res : "");
    return res;
}
string num_to_string(int num) {
    if (num < 0) return "Negative " + num_to_string(-1 * num);
    vector<string> v = {"Thousand", "Million", "Billion"};
    string res = convert_hundred(num % 1000);
    for (int i = 0; i < 3; ++i) {
        num /= 1000;
        res = num % 1000 ? convert_hundred(num % 1000) + " " + v[i] + " " + res : res;
    }
    while (res.back() == ' ') res.pop_back();
    return res;
}