              self.last = node
  
      def dequeue(self):
          # Empty list
          if self.first is None and self.last is None:
              return None
          data = self.first.data
          # Remove only element from a one element list
          if self.first == self.last:
              self.first = None
 
</s> remove         data = self.first.data
 </s> add         data = self.head.data </s> remove         if self.first == self.last:
            self.first = None
            self.last = None
 </s> add         if self.head == self.tail:
            self.head = None
            self.tail = None </s> remove     "        data = self.first.data\n",
 </s> add     "        data = self.head.data\n", </s> remove     "        if self.first is None and self.last is None:\n",
 </s> add     "        if self.head is None and self.tail is None:\n", </s> remove             self.last.next = node
            self.last = node
 </s> add             self.tail.next = node
            self.tail = node
        if self.head is None and self.tail is None:
              self.last = node
  
      def dequeue(self):
          # Empty list
        if self.head is None and self.tail is None:
              return None
          data = self.first.data
          # Remove only element from a one element list
          if self.first == self.last:
              self.first = None
keep keep keep keep 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