def helper(n, largest):
	"""
	:param n: int,
	:param largest: int, to find the biggest digit
	:return: int, the biggest digit in n
	Because digit < 10, this function recursively check every digit of n
	"""
	remainder = n % 10
	if n < 10:  # Base case!
		if remainder > largest:
			return remainder
		else:
			return largest
	elif remainder > largest:  # Recursive
		largest = remainder
		return helper((n-remainder)//10, largest)
	else:  # Recursive
		return helper((n-remainder)//10, largest)