504. Base 7

class Solution:
    def convertToBase7(self, num):
        """
        :type num: int
        :rtype: str
        """
        divider = num
        def calculation (divider):
            number = ""
            while divider > 0:
                remainder = divider % 7
                divider = divider // 7
                number =str(remainder) + number
            return number
        if divider == 0:
            return "0"
        if divider < 0:
            return "-" + calculation (-divider)
        if divider > 0:
            return calculation (divider)