18.Base 7

class Solution {
public:
    string convertTo7(int num) {
        string sign = "";
        string rst = "";
        if (num < 0) {
            num = -num;
            sign = "-";
        }
        while (num >= 7) {
            int left = num % 7;
            num = num / 7;
            rst = to_string(left) + rst;
        }
        return sign + to_string(num) + rst;
    }
};