Define n!! as

n!! = 1 \* 3 \* 5 \* ... \* n  if n is odd, 

n!! = 2 \* 4 \* 6 \* ... \* n  if n is even. 

Hence 8!! = 2 \* 4 \* 6 \* 8 = 384, there is no zero at the end. 
30!! has 3 zeros at the end. 

For a positive integer n, please count how many zeros are there at 
the end of n!!. 

Example: 

count\_zeros\_n\_double\_fact(30) should return 3

Difficulty: EASY
Starter code: def count_zeros_n_double_fact(n):
	

Solutions:
def count_zeros_n_double_fact(n):
	if n % 2 != 0:
		return 0
	k = 0
	while n >= 10:
		k += n // 10
		n //= 5
	return k


def count_zeros_n_double_fact(n):
	return 0 if n % 2 == 1 else n // 6250 + n // 1250 + n // 250 + n // 50 + n // 10


from functools import reduce

def count_zeros_n_double_fact(n):
	df = n % 2 or reduce(int.__mul__, range(2, n + 1, 2), 1)
	return next((i for (i, d) in enumerate(f'{df}'[::-1]) if d != '0'))


from math import log

def count_zeros_n_double_fact(n):
	return 0 if n & 1 else sum((n // 2 // 5 ** x for x in range(1, int((log(n) - log(2)) / log(5)) + 1)))


def count_zeros_n_double_fact(n):
	return 0 if n % 2 else sum((n // (10 * 5 ** x) for x in range(len(str(n)) + 1)))


def count_zeros_n_double_fact(n):
	if n % 2:
		return 0
	(s, n) = (0, n // 2)
	while n >= 5:
		n //= 5
		s += n
	return s


def count_zeros_n_double_fact(n):
	if n % 2 == 1:
		return 0
	res = 0
	working = n // 2
	while working > 0:
		res += working // 5
		working //= 5
	return res


def count_zeros_n_double_fact(n):
	if n % 2 == 1:
		return 0
	return n // 10 + n // 50 + n // 250


def count_zeros_n_double_fact(n):
	fact = 1
	while n > 2:
		fact *= n
		n -= 2
	return len(str(fact)) - len(str(fact).rstrip('0'))