      "        if self.first is None and self.last is None:\n",
      "            self.first = node\n",
      "            self.last = node\n",
      "        else:\n",
      "            self.last.next = node\n",
      "            self.last = node\n",
      "\n",
      "    def dequeue(self):\n",
      "        # Empty list\n",
      "        if self.first is None and self.last is None:\n",
      "            return None\n",
 
</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     "        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     "        data = self.first.data\n",
 </s> add     "        data = self.head.data\n", </s> remove     "        self.first = None\n",
    "        self.last = None\n",
 </s> add     "        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.tail.next = node\n",
    "            self.tail = node\n",
      "        if self.first is None and self.last is None:\n",
      "            self.first = node\n",
      "            self.last = node\n",
      "        else:\n",
    "            self.tail.next = node\n",
    "            self.tail = node\n",
    "            self.tail.next = node\n",
    "            self.tail = node\n",
      "\n",
      "    def dequeue(self):\n",
      "        # Empty list\n",
      "        if self.first is None and self.last is None:\n",
      "            return None\n",
keep keep keep keep 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_solution.ipynb