def find_largest_digit_helper(n, l):
	"""
	helper function
	base case: n//10 == 0
	n//10 => remove the units digit of n
	n%10 => get the units digit of n
	:param n: (int) the number which units digit are removing.
	:param l: (int) the stored largest digit so far
	"""
	if n//10 == 0:  # base case: no more digits after removal of units digit
		if n % 10 > l:  # if this number is larger than the largest digit so far.
			l = n % 10
		return l
	else:  # recursive case: still have more than 1 digit.
		if n % 10 > l:  # if this number is larger than the largest digit so far.
			l = n % 10
		return find_largest_digit_helper(n//10, l)