504. 七进制数（进制转换模板）

class Solution:
    def convertToBase7(self, num: int) -> str:
        res = ''
        if num == 0:
            return '0'
        if num < 0:
            a = '-'
            num = -num
        else:
            a =''
        while num:
            res += str(num % 7)
            num //= 7
        return a + res[::-1]