    }
   ],
   "metadata": {
    "kernelspec": {
     "display_name": "Python 2",
     "language": "python",
     "name": "python2"
    },
    "language_info": {
     "codemirror_mode": {
 
</s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "name": "python2"
 </s> add    "name": "python3" </s> remove    "name": "python2"
 </s> add    "name": "python3" </s> remove     "version": 2
 </s> add     "version": 3 </s> remove     "version": 2
 </s> add     "version": 3
   "display_name": "Python 3",
    }
   ],
   "metadata": {
    "kernelspec": {
   "display_name": "Python 3",
     "language": "python",
     "name": "python2"
    },
    "language_info": {
     "codemirror_mode": {
keep keep keep keep replace keep keep keep keep keep
 Polish quicksort challenge and solution

Add None input test case.  Remove difficult to read 'Pythonic' solution.  Update space complexity discussion. 
sorting_searching/quick_sort/quick_sort_challenge.ipynb