def helper_find_largest_digit(n, largest):
	"""
	This function is the helper function of find_largest_digit(n)
	:param n: int, given in main function
	:param largest: int, the latest largest digit in n
	:return: the largest digit in n
	"""
	# check whether n is positive or negative
	if n < 0:
		n = - n

	# check if the last digit in n is the largest digit
	last = n % 10
	if last > largest:
		largest = last

	# check if n is single digit; if not (=else), remove last digit and do recursion
	if n // 10 == 0:
		return largest
	else:
		return helper_find_largest_digit(n // 10, largest)