def find_largest_digit_helper(n, max_digit_now):
	"""
	:param n: int, the number which user wants to find max digit now
	:param max_digit_now: int, the max digit now
	:return: int, the max digit of n
	Reduce one digit in each recursive case to approach base case(n<10)
	"""
	if n < 0:
		# Negative number, should plus -1 to be positive
		n = -n
	if n % 10 > max_digit_now:
		# Explore and assign right max_digit
		max_digit_now = n % 10
	if n < 10:
		# Base Case
		return max_digit_now
	else:
		# Recursive Case
		return find_largest_digit_helper(n // 10, max_digit_now)