      "        src = Stack()\n",
      "        buff = Stack()\n",
      "        dest = Stack()\n",
      "\n",
      "        print('Test: NULL towers')\n",
      "        hanoi(num_disks, None, None, None)\n",
      "\n",
      "        print('Test: 0 disks')\n",
      "        hanoi(num_disks, src, dest, buff)\n",
      "        assert_equal(dest.pop(), None)\n",
 
</s>
    "        print('Test: None towers')\n",
      "        src = Stack()\n",
      "        buff = Stack()\n",
      "        dest = Stack()\n",
      "\n",
    "        print('Test: None towers')\n",
      "        hanoi(num_disks, None, None, None)\n",
      "\n",
      "        print('Test: 0 disks')\n",
      "        hanoi(num_disks, src, dest, buff)\n",
      "        assert_equal(dest.pop(), None)\n",
keep keep keep keep replace keep keep keep keep keep
 Changed discussions of NULL to None to be more Pythonic.
stacks_queues/hanoi/hanoi_solution.ipynb