{
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  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Solution Notebook"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Problem: Find the Difference.\n",
    "\n",
    "See the [LeetCode](https://leetcode.com/problems/find-the-difference/) problem page.\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 strings are ASCII?\n",
    "    * Yes\n",
    "* Is case important?\n",
    "    * The strings are lower case\n",
    "* Can we assume the inputs are valid?\n",
    "    * No, check for None\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* None input -> TypeError\n",
    "* 'aaabbcdd', 'abdbacade' -> 'e'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "* Keep a dictionary of seen values in s\n",
    "* Loop through t, decrementing the seen values\n",
    "    * If the char is not there or if the decrement results in a negative value, return the char\n",
    "\n",
    "Complexity:\n",
    "* Time: O(m+n), where m and n are the lengths of s, t\n",
    "* Space: O(h), for the dict, where h is the unique chars in s"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def find_diff(self, s, t):\n",
    "        if s is None or t is None:\n",
    "            raise TypeError('s or t cannot be None')\n",
    "        seen = {}\n",
    "        for char in s:\n",
    "            if char in seen:\n",
    "                seen[char] += 1\n",
    "            else:\n",
    "                seen[char] = 1\n",
    "        for char in t:\n",
    "            try:\n",
    "                seen[char] -= 1\n",
    "            except KeyError:\n",
    "                return char\n",
    "            if seen[char] < 0:\n",
    "                return char\n",
    "        return None"
   ]
  },
  {
   "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_str_diff.py\n"
     ]
    }
   ],
   "source": [
    "%%writefile test_str_diff.py\n",
    "from nose.tools import assert_equal, assert_raises\n",
    "\n",
    "\n",
    "class TestFindDiff(object):\n",
    "\n",
    "    def test_find_diff(self):\n",
    "        solution = Solution()\n",
    "        assert_raises(TypeError, solution.find_diff, None)\n",
    "        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')\n",
    "        print('Success: test_find_diff')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestFindDiff()\n",
    "    test.test_find_diff()\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_find_diff\n"
     ]
    }
   ],
   "source": [
    "%run -i test_str_diff.py"
   ]
  }
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