504. Base 7

class Solution {
public:
    string convertToBase7(int num) {
        if (num == 0)   return "0";
        
        long n = abs(num);
        int flag = num > 0 ? 1 : -1;
        
        string res;
        while (n) {
            res += to_string(n % 7);
            n /= 7;
        }
        if (flag == -1) res.push_back('-');
        reverse(res.begin(), res.end());
        return res;
    }
};