​LeetCode刷题实战504：七进制数

示例 1:
输入: num = 100
输出: "202"

示例 2:
输入: num = -7
输出: "-10"

class Solution {
public:
    string convertToBase7(int num) {
      if(num == 0)
        return "0";
        bool negative = (num < 0);
        num = abs(num);
        string ans;
        while(num)
        {
          ans.append(to_string(num%7));//余数
          num /= 7;
        }
        if(negative)
          ans.push_back('-');
        reverse(ans.begin(), ans.end());//逆序
        return ans;
    }
};