{
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   "metadata": {},
   "source": [
    "<small><i>This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).</i></small>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Solution Notebook"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3BC2D4'.  Only compress the string if it saves space.\n",
    "\n",
    "* [Constraints](#Constraints)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Constraints\n",
    "\n",
    "* Can we assume the string is ASCII?\n",
    "    * Yes\n",
    "    * Note: Unicode strings could require special handling depending on your language\n",
    "* Is this case sensitive?\n",
    "    * Yes\n",
    "* Can you use additional data structures?  \n",
    "    * Yes\n",
    "* Can we assume this fits in memory?\n",
    "    * Yes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* None -> None\n",
    "* '' -> ''\n",
    "* 'AABBCC' -> 'AABBCC'\n",
    "* 'AAABCCDDDD' -> 'A3BC2D4'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "* For each char in string\n",
    "    * If char is the same as last_char, increment count\n",
    "    * Else\n",
    "        * Append last_char and count to compressed_string\n",
    "        * last_char = char\n",
    "        * count = 1\n",
    "* Append last_char and count to compressed_string\n",
    "* If the compressed string size is < string size\n",
    "    * Return compressed string\n",
    "* Else\n",
    "    * Return string\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: O(n)\n",
    "\n",
    "Complexity Note:\n",
    "* Although strings are immutable in Python, appending to strings is optimized in CPython so that it now runs in O(n) and extends the string in-place.  Refer to this [Stack Overflow post](http://stackoverflow.com/a/4435752)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def compress_string(string):\n",
    "    if string is None or len(string) == 0:\n",
    "        return string\n",
    "    result = ''\n",
    "    prev_char = string[0]\n",
    "    count = 0\n",
    "    for char in string:\n",
    "        if char == prev_char:\n",
    "            count += 1\n",
    "        else:\n",
    "            result += prev_char + (str(count) if count > 1 else '')\n",
    "            prev_char = char\n",
    "            count = 1\n",
    "    result += prev_char + (str(count) if count > 1 else '')\n",
    "    return result if len(result) < len(string) else string"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Overwriting test_compress.py\n"
     ]
    }
   ],
   "source": [
    "%%writefile test_compress.py\n",
    "from nose.tools import assert_equal\n",
    "\n",
    "\n",
    "class TestCompress(object):\n",
    "\n",
    "    def test_compress(self, func):\n",
    "        assert_equal(func(None), None)\n",
    "        assert_equal(func(''), '')\n",
    "        assert_equal(func('AABBCC'), 'AABBCC')\n",
    "        assert_equal(func('AAABCCDDDDE'), 'A3BC2D4E')\n",
    "        print('Success: test_compress')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestCompress()\n",
    "    test.test_compress(compress_string)\n",
    "\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    main()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Success: test_compress\n"
     ]
    }
   ],
   "source": [
    "%run -i test_compress.py"
   ]
  }
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