     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
      "* NULL tower(s)\n",
      "* 0 disks\n",
      "* 1 disk\n",
      "* 2 or more disks"
     ]
    },
 
</s>
    "* None tower(s)\n",
     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
    "* None tower(s)\n",
      "* 0 disks\n",
      "* 1 disk\n",
      "* 2 or more disks"
     ]
    },
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