Base 7

纯数学，判断好0就行

class Solution(object):
    def convertToBase7(self, num):
        """
        :type num: int
        :rtype: str
        """
        if not num:
            return '0'

        sign = 1 if num >= 0 else -1
        num = abs(num)
        result = ''
        while num:
            num, digit = divmod(num, 7)
            result = `digit` + result
        return result if sign == 1 else '-' + result