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    "## Problem: Implement quick sort.\n",
    "\n",
    "* [Clarifying Questions](#Clarifying-Questions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Pythonic-Code](#Pythonic-Code)"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Clarifying Questions\n",
    "\n",
    "* Is a naiive quicksort (ie not in-place) ok?\n",
    "    * Yes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* Empty input\n",
    "* One element\n",
    "* Two or more elements"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "Wikipedia's animation:\n",
    "![alt text](http://upload.wikimedia.org/wikipedia/commons/6/6a/Sorting_quicksort_anim.gif)\n",
    "\n",
    "* Set pivot to the middle element in the data\n",
    "* For each element:\n",
    "    * If current element is the pivot, continue\n",
    "    * If the element is less than the pivot, add to left array\n",
    "    * Else, add to right array\n",
    "* Recursively apply quicksort to the left array\n",
    "* Recursively apply quicksort to the right array\n",
    "* Merge the left array + pivot + right array\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n log(n)) average, best, O(n^2) worst\n",
    "* Space: O(n), n extra space, n recursion depth, generally not stable"
   ]
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   "metadata": {},
   "source": [
    "## Code"
   ]
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   "source": [
    "def quick_sort(data):\n",
    "    if len(data) < 2:\n",
    "        return data\n",
    "    left = []\n",
    "    right = []\n",
    "    pivot_index = len(data) / 2\n",
    "    pivot_value = data[pivot_index]\n",
    "    for i in xrange(0, len(data)):\n",
    "        if i == pivot_index:\n",
    "            continue\n",
    "        if data[i] < pivot_value:\n",
    "            left.append(data[i])\n",
    "        else:\n",
    "            right.append(data[i])\n",
    "    left = quick_sort(left)\n",
    "    right = quick_sort(right)\n",
    "    return left + [pivot_value] + right"
   ]
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   "source": [
    "print('Empty input')\n",
    "data = []\n",
    "print(quick_sort(data))\n",
    "print('One element')\n",
    "data = [5]\n",
    "print(quick_sort(data))\n",
    "print('Two or more elements')\n",
    "data = [5, 1, 7, 2, 6, -3, 5, 7, -1]\n",
    "print(quick_sort(data))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pythonic-Code\n",
    "\n",
    "From http://stackoverflow.com/questions/18262306/quick-sort-with-python"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
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   "outputs": [],
   "source": [
    "def quick_sort(arr): \n",
    "     if len(arr) <= 1:\n",
    "          return arr\n",
    "     else:\n",
    "          return qsort([x for x in arr[1:] if x<arr[0]]) + [arr[0]] + qsort([x for x in arr[1:] if x>=arr[0]])"
   ]
  }
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