LeetCode 504. Base 7

Given an integer, return its base 7 string representation.

Example 1:
Input: 100
Output: "202"

Example 2:
Input: -7
Output "-10"

Note: The input will be in range of [-1e7, 1e7].

class Solution { //水题
public:
    string convertToBase7(int num) {
        string ans;
        int flag=0;
        if(num<0)
            flag=1;
        else if(num==0)
            ans.insert(ans.begin(),1,'0');
        num=abs(num);
        while(num){
            ans.insert(ans.begin(),1,'0'+num%7);
            num/=7;
        }
        if(flag)
            ans.insert(ans.begin(),1,'-');
        return ans;
    }
};