     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
      "* Empty input -> []\n",
      "* One element -> [element]\n",
      "* Two or more elements\n",
      "* Left and right subarrays of different lengths"
 
</s> remove     "* Space: O(n), stable"
 </s> add     "* Space: O(n+m), n = number of elements, m = recursion depth\n",
    "\n",
    "Most implementations are stable." </s> remove     "    if len(data) < 2:\n",
 </s> add     "    if data is None or len(data) < 2:\n",
    "* None -> None\n",
     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
    "* None -> None\n",
      "* Empty input -> []\n",
      "* One element -> [element]\n",
      "* Two or more elements\n",
      "* Left and right subarrays of different lengths"
keep keep keep add keep keep keep keep
 Polish merge sort challenge and solution

Add None input test case.  Update space complexity discussion. 
sorting_searching/merge_sort/merge_sort_challenge.ipynb