     "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     "    right = data[mid:len(data)]\n",
 </s> add     "    right = data[mid:]\n", </s> remove     "    if len(data) < 2:\n",
 </s> add     "    if data is None or len(data) < 2:\n",
    "* None -> None\n",
     "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 add keep 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_solution.ipynb