示例
样例 1:
输入: num = 100
输出: 202

样例 2:
输入: num = -7
输出: -10

class Solution {
    
    
public:
    /**
     * @param num: the given number
     * @return: The base 7 string representation
     */
    string convertToBase7(int num) {
    
    
        // Write your code here
        bool negative = num < 0;
        if(negative)
        	num = -num;
        string ans;
        int base = 7;
        do
        {
    
    
        	ans += (num%base)+'0';
        	num /= base;
        }while(num);
        reverse(ans.begin(), ans.end());
        if(negative)
        	ans = "-"+ans;
        return ans;
    }
};