給定一個整數，將其轉化為7進位制，並以字串形式輸出。

示例 1:

輸入: 100
輸出: "202"
示例 2:

輸入: -7
輸出: "-10"
注意: 輸入範圍是[-1e7, 1e7] 。

class Solution {
public:
    string convertToBase7(int num) {
        if (num == 0) return "0";
        if (num < 0) return "-" + convertToBase7(-num);
        return (num >= 7 ? convertToBase7(num / 7) : ""
 
) + (char)(48 + num % 7);
    }
};

class Solution {
public:
    string convertToBase7(int num) {
        if (num == 0) return "0";
        string res = "";
        string sgn = "";
        if (num < 0) {
            num = -num;
            sgn = "-";
        }
        while (num > 0) {
            res 
 
= std::to_string(num % 7)+res;
            num /= 7;
        }
        return sgn + res;
    }
};

class Solution {
public:
    string convertToBase7(int num) {
        if (num == 0) return "0";
        string res = "";
        string sgn = "";
        if (num < 0) {
            num = -num;
            sgn 
 
= "-";
        }
        while (num > 0) {
            res.push_back(48 + num % 7);
            num /= 7;
        }
        std::reverse(begin(res), end(res));
        return sgn + res;
    }
};