def helper(n, order, current_max):
	"""
	This function help find the biggest digit.
	:param n: the input integer
	:param order: the number of digits
	:param current_max: the currently biggest digit
	:return: the biggest digit
	"""
	# +-(0~9)
	if order == 0:
		return int(current_max)
	# +-(10~99999)
	else:
		# Get digit
		division = 10**(order-1)
		num = (n - (n % division))/division
		if num > current_max:
			current_max = num
		return helper(n % division, order-1, current_max)