七进制

给定一个整数，将其转化为7进制，并以字符串形式输出。

示例 1:

输入: 100
输出: "202"
示例 2:

输入: -7
输出: "-10"

class Solution {
    public String convertToBase7(int num) {
        StringBuffer buf = new StringBuffer();
        int yu = 0;
        boolean f = false;
        if (num < 0) {
            num = -1 * num;
            f = true;
        }
        while (num >= 7) {
            yu = num % 7;
            num = num / 7;
            buf.append(yu);
        }
        buf.append(num);
        String str = new String(buf.reverse());
        if (f) {
            return "-" + str;
        }
        return str;
        
    }
}