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

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