leetcode 504. Base 7

转成7进制，辗转相除。

    string convertToBase7(int num) {
        if (num == 0)
            return "0";
        bool neg = false;
        if (num < 0) {
            neg = true;
            num = -num;
        }
        string ret;
        
        int rem;
        while (num) {
            rem = num % 7;
            num = num / 7;
            ret.push_back(rem + '0');
        }
        reverse(ret.begin(), ret.end());
        if (neg)
            ret = "-" + ret;
        return ret;
    }