interactive-coding-challenges/graphs_trees/invert_tree/invert_tree_solution.ipynb

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   "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)."
   ]
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   "source": [
    "# Solution Notebook"
   ]
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   "metadata": {},
   "source": [
    "## Problem: Invert a binary tree.\n",
    "\n",
    "* [Constraints](#Constraints)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Constraints\n",
    "\n",
    "* What does it mean to invert a binary tree?\n",
    "    * Swap all left and right node pairs\n",
    "* Can we assume we already have a Node class?\n",
    "    * Yes\n",
    "* Can we assume the inputs are valid?\n",
    "    * No\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "<pre>\n",
    "Input:\n",
    "     5\n",
    "   /   \\\n",
    "  2     7\n",
    " / \\   / \\\n",
    "1   3 6   9\n",
    "\n",
    "Output:\n",
    "     5\n",
    "   /   \\\n",
    "  7     2\n",
    " / \\   / \\\n",
    "9   6 3   1\n",
    "</pre>"
   ]
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   "source": [
    "## Algorithm\n",
    "\n",
    "* Base case\n",
    "    * If the root is None, return\n",
    "* Recursive case\n",
    "    * Recurse on the left node\n",
    "    * Recurse on the right node\n",
    "    * Swap left and right\n",
    "* Return the node\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: O(h), where h is the height, for the recursion depth"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "%run ../bst/bst.py"
   ]
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  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
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   "outputs": [],
   "source": [
    "class InverseBst(Bst):\n",
    "\n",
    "    def invert_tree(self):\n",
    "        if self.root is None:\n",
    "            raise TypeError('root cannot be None')\n",
    "        return self._invert_tree(self.root)\n",
    "\n",
    "    def _invert_tree(self, root):\n",
    "        if root is None:\n",
    "            return\n",
    "        self._invert_tree(root.left)\n",
    "        self._invert_tree(root.right)\n",
    "        root.left, root.right = root.right, root.left\n",
    "        return root"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Overwriting test_invert_tree.py\n"
     ]
    }
   ],
   "source": [
    "%%writefile test_invert_tree.py\n",
    "from nose.tools import assert_equal, assert_raises\n",
    "\n",
    "\n",
    "class TestInvertTree(object):\n",
    "\n",
    "    def test_invert_tree(self):\n",
    "        root = Node(5)\n",
    "        bst = InverseBst(root)\n",
    "        node2 = bst.insert(2)\n",
    "        node3 = bst.insert(3)\n",
    "        node1 = bst.insert(1)\n",
    "        node7 = bst.insert(7)\n",
    "        node6 = bst.insert(6)\n",
    "        node9 = bst.insert(9)\n",
    "        result = bst.invert_tree()\n",
    "        assert_equal(result, root)\n",
    "        assert_equal(result.left, node7)\n",
    "        assert_equal(result.right, node2)\n",
    "        assert_equal(result.left.left, node9)\n",
    "        assert_equal(result.left.right, node6)\n",
    "        assert_equal(result.right.left, node3)\n",
    "        assert_equal(result.right.right, node1)\n",
    "        print('Success: test_invert_tree')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestInvertTree()\n",
    "    test.test_invert_tree()\n",
    "\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    main()"
   ]
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   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Success: test_invert_tree\n"
     ]
    }
   ],
   "source": [
    "%run -i test_invert_tree.py"
   ]
  }
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