      "    * Return result array\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n log(n))\n",
      "* Space: O(n), stable"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "    right = data[mid:len(data)]\n",
 </s> add     "    right = data[mid:]\n",
    "* Space: O(n+m), n = number of elements, m = recursion depth\n",
    "\n",
    "Most implementations are stable."
      "    * Return result array\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n log(n))\n",
    "* Space: O(n+m), n = number of elements, m = recursion depth\n",
    "\n",
    "Most implementations are stable."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace 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