def find_largest_digit_helper(n, greater):
	"""
	This function is the helper of find_largest_digit(n)
	:param n: int, the input number
	:param greater: int, the greater number within n
	:return: return greater
	"""
	if n != 0:
		if n % 10 > greater:
			greater = n % 10
		return find_largest_digit_helper(n//10, greater)
	return greater