[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].

string convertToBase7(int num)
     {
         if (num == ) return "";
         string ret = "";
         int temp = num;
         if (num < ) num = -num;
         while (num)
         {
             ret.insert(ret.begin(), num %  + '');
            //ret = to_string(num % 7) + ret;//to_string用法
             num /= ;
         }
         if (temp < )ret.insert(ret.begin(), '-');
         return ret;
     }