504 base 7

class (object):
    def convertToBase7(self, num):
        """
        :type num: int
        :rtype: str
        """
        
        isNeg = False
        if (num < 0):
            num = -num
            isNeg = True
        elif (num == 0):
            return "0"
        res = ""
        while (num):
            tmp = num % 7
            res = str(tmp) + res
            num = num / 7
        if (isNeg):
            return "-" + res
        else:
            return res