      "    right = quick_sort(right)\n",
      "    return left + [pivot_value] + right"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Pythonic-Code\n",
      "\n",
      "The following code from [Stack Overflow](http://stackoverflow.com/questions/18262306/quick-sort-with-python) is very concise, although it might be a little difficult to read:"
     ]
    },
    {
     "cell_type": "code",
     "execution_count": 2,
     "metadata": {
      "collapsed": true
     },
     "outputs": [],
     "source": [
      "def quick_sort_alt(arr):\n",
      "    if len(arr) <= 1:\n",
      "        return arr\n",
      "    else:\n",
      "        return quick_sort_alt([x for x in arr[1:] if x < arr[0]]) + [arr[0]] + quick_sort_alt([x for x in arr[1:] if x >= arr[0]])"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Unit Test\n",
 
</s> remove    "execution_count": 3,
 </s> add    "execution_count": 2, </s> remove     "    if len(data) < 2:\n",
 </s> add     "    if data is None or len(data) < 2:\n", </s> remove    "execution_count": 4,
 </s> add    "execution_count": 3, </s> remove     "* Space: O(n), n extra space, n recursion depth, generally not stable"
 </s> add     "* Space: O(n+m), n = number of elements, m = recursion depth\n",
    "\n",
    "Most implementations are not stable."
      "    right = quick_sort(right)\n",
      "    return left + [pivot_value] + right"
     ]
    },
























    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Unit Test\n",
keep keep keep keep replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace 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_solution.ipynb