我用的遞推求階乘，str.count('0')的方法計算。但超過題目要求的O(log n)了。

所以， 最後這道題是到數學題，因式分解。。。數學推導過程

class Solution(object):
    def trailingZeroes(self, n):
        """
        :type n: int
        :rtype: int
        """
        r = 0
        while n >= 5:
            n = n // 5
            r += n
        return r

順便複習一下，常規求階乘。

class Solution(object):
    def trailingZeroes(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n == 0:
            return 0
        result = n
        for i in range(1,n):
            result *= i
        count_0 = 0
        num = str(result)
        num = num[::-1]
        # print(num)
        for i in range(len(num)):
            if num[i] == '0':
                count_0 += 1
            else:
                return count_0



    # def factorial(self,n):
    #     if n==1:
    #         return 1
    #     else:
    #         return n*factorial(n-1)