def find_largest_digit_helper(num, max_num):
	"""
	:param num: the number that we should pick the largest digit from.
	:param max_num: current maximum number
	:return: the largest number
	"""
	# there is no number left
	if num == 0:
		return max_num

	else:
		# pick the last number to compare
		if num % 10 > max_num:
			max_num = num % 10
			# delete the last number and compare again
			return find_largest_digit_helper(num//10, max_num)
		else:
			return find_largest_digit_helper(num//10, max_num)