      "collapsed": false
     },
     "outputs": [],
     "source": [
      "def merge_sort(data):\n",
      "    # TODO: Implement me\n",
      "    pass"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "from nose.tools import assert_equal\n",
 </s> add     "from nose.tools import assert_equal, assert_raises\n", </s> remove     "            \n",
    "    # Copy remaining elements\n",
    "    while l < len(left):\n",
    "        result.append(left[l])\n",
    "        l += 1\n",
    "    while r < len(right):\n",
    "        result.append(right[r])\n",
    "        r += 1\n",
    "    return result\n",
    "\n",
    "\n",
    "def merge_sort(data):\n",
    "    if data is None or len(data) < 2:\n",
    "        return data\n",
    "    mid = len(data) // 2\n",
    "    left = data[:mid]\n",
    "    right = data[mid:]\n",
    "    left = merge_sort(left)\n",
    "    right = merge_sort(right)\n",
    "    return merge(left, right)"
 </s> add     "        return result" </s> remove     "* None -> None\n",
 </s> add     "* None -> Exception\n",
    "class MergeSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass"
      "collapsed": false
     },
     "outputs": [],
     "source": [
    "class MergeSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass"
    "class MergeSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass"
    "class MergeSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        # TODO: Implement me\n",
    "        pass"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace replace replace keep keep keep keep keep
 Update merge sort challenge (#130)

Update constraints, test cases, code, and tests. 
sorting_searching/merge_sort/merge_sort_challenge.ipynb