      "\n",
      "* None -> None\n",
      "* '' -> ''\n",
      "* 'AABBCC' -> 'AABBCC'\n",
      "* 'AAABCCDDDD' -> 'A3B1C2D4'"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "## Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3B1C2D4'.  Only compress the string if it saves space.\n",
 </s> add     "## Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3BCCD4'.  Only compress the string if it saves space.\n", </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "name": "python2"
 </s> add    "name": "python3" </s> remove     "version": 2
 </s> add     "version": 3 </s> remove     "        assert_equal(func('AAABCCDDDD'), 'A3B1C2D4')\n",
 </s> add     "        assert_equal(func('AAABCCDDDD'), 'A3BCCD4')\n",
    "* 'AAABCCDDDD' -> 'A3BCCD4'"
      "\n",
      "* None -> None\n",
      "* '' -> ''\n",
      "* 'AABBCC' -> 'AABBCC'\n",
    "* 'AAABCCDDDD' -> 'A3BCCD4'"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace keep keep keep keep keep
 Added the same changes to test cases in the challenge notebook 
arrays_strings/compress/compress_challenge.ipynb