queue.py

class node:
	'''node for linked list type structures'''
	def __init__(self, val):
		self.val = val
		self.next = None
		self.prev = None
	
	def __repr__(self):
		return str(self.val)

class queueLL:
	'''queue data structure (First in First out) based on linked lists'''
	def __init__(self):
		self.last = None
		self.first = None
		self.size = 0
	
	def add(self, val):
		newNode = node(val)
		#if the queue is empty, set the new node as a the first item
		if self.first == None:
			self.first = newNode
		#the next node after the new node is always the current last node, even if the list is empty and the last item is none
		newNode.next = self.last

		#if there is no last node (empty queue), skip this. 
		#Otherwise set the current last node's previous value to the new node.
		if self.last != None:
			self.last.prev = newNode
		#the last item in the queue is always the new node
		self.last = newNode
		self.size += 1

	def get(self):
		#skip if the queue is empty
		if self.first == None:
			return None
		else:
			#return the first item in the queue, reset the first item to the next
			nextNode = self.first
			self.first = nextNode.prev
			self.size -= 1
			return nextNode.val

class queueL:
	'''queue data structure (First in First out) based on lists / arrays'''
	def __init__(self):
		self.items = []
	def add(self, x):
		self.items.append(x)
	def get(self):
		if len(self.items) == 0:
			return None
		item = self.items[0]
		del self.items[0]
		return item
	def peek(self):
		return self.items[0]
	def __len__(self):
		return len(self.items)
	def __repr__(self):
		return str(self.items)