      "        if self.first == self.last:\n",
      "            self.first = None\n",
      "            self.last = None\n",
      "        else:\n",
      "            self.first = self.first.next\n",
      "        return data"
     ]
    },
    {
     "cell_type": "code",
 
</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     "        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\n",
    "            self.last = node\n",
 </s> add     "            self.tail.next = node\n",
    "            self.tail = node\n", </s> remove             self.first = self.first.next
 </s> add             self.head = self.head.next
    "            self.head = self.head.next\n",
      "        if self.first == self.last:\n",
      "            self.first = None\n",
      "            self.last = None\n",
      "        else:\n",
    "            self.head = self.head.next\n",
      "        return data"
     ]
    },
    {
     "cell_type": "code",
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_solution.ipynb