          self.last = None
  
      def enqueue(self, data):
          node = Node(data)
          if self.first is None and self.last is None:
              self.first = node
              self.last = node
          else:
              self.last.next = node
              self.last = node
  
      def dequeue(self):
 
</s> remove             self.last.next = node
            self.last = node
 </s> add             self.tail.next = node
            self.tail = node </s> remove         self.first = None
        self.last = None
 </s> add         self.head = None
        self.tail = None </s> remove         if self.first is None and self.last is None:
 </s> add         if self.head is None and self.tail is None: </s> remove     "        if self.first is None and self.last is None:\n",
    "            self.first = node\n",
    "            self.last = node\n",
 </s> add     "        # Empty list\n",
    "        if self.head is None and self.tail is None:\n",
    "            self.head = node\n",
    "            self.tail = node\n", </s> remove     "        self.first = None\n",
    "        self.last = None\n",
 </s> add     "        self.head = None\n",
    "        self.tail = None\n",
        # Empty list
        if self.head is None and self.tail is None:
            self.head = node
            self.tail = node
          self.last = None
  
      def enqueue(self, data):
          node = Node(data)
        # Empty list
        if self.head is None and self.tail is None:
            self.head = node
            self.tail = node
        # Empty list
        if self.head is None and self.tail is None:
            self.head = node
            self.tail = node
        # Empty list
        if self.head is None and self.tail is None:
            self.head = node
            self.tail = node
          else:
              self.last.next = node
              self.last = node
  
      def dequeue(self):
keep keep keep keep replace replace replace keep keep keep keep keep
 Polish queue challenge and solution (#70)

Update constraints, algorithm discussion, and code. 
stacks_queues/queue_list/queue_list.py