LeetCode "Integer to English Words"

Just take care of corner cases.

vector<string> sec3 = { "", "Thousand", "Million", "Billion" };
vector<string> sig = { "", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
vector<string> teen = { "", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen",
"Seventeen", "Eighteen", "Nineteen" };
vector<string> ty = { "", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };

class Solution
{
    string sec3to(int num)
    {
        int a = num % 10;
        int b = (num / 10) % 10;
        int c = (num / 100) % 10;

        string ret;
        if (b == 1 && a != 0) // teens
        {
            ret += teen[a];
        }
        else if (a == 0 && b != 0) // ty-s
        {
            ret += ty[b];
        }
        else // all other
        {
            if (b > 0) ret += ty[b] + " ";
            if (a > 0) ret += sig[a];
        }
        if (c > 0)
        {
            string hstr = sig[c] + " Hundred";
            if (ret.empty())
                ret = hstr;
            else
                ret = hstr + " " + ret;
        }

        return ret;
    }
public:
    string numberToWords(int num)
    {
        if (num == 0) return "Zero";
        string ret;

        int sec = num % 1000;
        int seci = 0;
        while (num)
        {
            string secstr = sec3to(sec);
            if (!secstr.empty())
                ret = secstr + (seci > 0 ? (" " + sec3[seci] + (ret.empty() ? "" : " " + ret)) : "");

            //
            num /= 1000;
            sec = num % 1000;
            seci++;
        }

        return ret;
    }
};