leetcode 504 base 7

class Solution {
public:
    string convertToBase7(int num) {
        if(num == 0) return "0";
        int a = num > 0 ? num : -num;
        int b;
        stack<int> sk;
        while(a != 0)
        {
            b = a % 7;
            a = a / 7;
            sk.push(b);
        }
        string res;
        while(!sk.empty())
        {
            res += to_string(sk.top());
            sk.pop();
        }
        return num > 0 ? res : "-" + res;
    }
};