Add invert tree challenge

graphs_trees/invert_tree/invert_tree_challenge.ipynb

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+{
+ "cells": [
+  {
+   "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": [
+    "# Challenge Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "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)\n",
+    "* [Solution Notebook](#Solution-Notebook)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "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"
+   ]
+  },
+  {
+   "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>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "Refer to the [Solution Notebook]().  If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%run ../bst/bst.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class InverseBst(Bst):\n",
+    "\n",
+    "    def invert_tree(self):\n",
+    "        # TODO: Implement me\n",
+    "        pass"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**The following unit test is expected to fail until you solve the challenge.**"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "# %load test_invert_tree.py\n",
+    "from nose.tools import assert_equal\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()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Solution Notebook\n",
+    "\n",
+    "Review the [Solution Notebook]() for a discussion on algorithms and code solutions."
+   ]
+  }
+ ],
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