interactive-coding-challenges/linked_lists/palindrome/palindrome_solution.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This notebook was prepared by [Donne Martin](http://donnemartin.com). 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: Determine if a linked list is a palindrome.\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 this is a non-circular, singly linked list?\n",
    "    * Yes\n",
    "* Is a single character or number a palindrome?\n",
    "    * No\n",
    "* Can we assume we already have a linked list class that can be used for this problem?\n",
    "    * Yes\n",
    "* Can we 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",
    "\n",
    "* Empty list -> False\n",
    "* Single element list -> False\n",
    "* Two or more element list, not a palindrome -> False\n",
    "* General case: Palindrome with even length -> True\n",
    "* General case: Palindrome with odd length -> True"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "* Reverse the linked list\n",
    "    * Iterate through the current linked list\n",
    "        * Insert to front the current node into a new linked list\n",
    "* Compare the reversed list with the original list\n",
    "    * Only need to compare the first half\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: O(n)\n",
    "\n",
    "Note:\n",
    "* We could also do this iteratively, using a stack to effectively reverse the first half of the string."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "%run ../linked_list/linked_list.py"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "from __future__ import division\n",
    "\n",
    "\n",
    "class MyLinkedList(LinkedList):\n",
    "\n",
    "    def is_palindrome(self):\n",
    "        if self.head is None or self.head.next is None:\n",
    "            return False\n",
    "        curr = self.head\n",
    "        reversed_list = MyLinkedList()\n",
    "        length = 0\n",
    "\n",
    "        # Reverse the linked list\n",
    "        while curr is not None:\n",
    "            reversed_list.insert_to_front(curr.data)\n",
    "            length += 1\n",
    "            curr = curr.next\n",
    "\n",
    "        # Compare the reversed list with the original list\n",
    "        # Only need to compare the first half\n",
    "        iterations = length // 2\n",
    "        curr = self.head\n",
    "        curr_reversed = reversed_list.head\n",
    "        for _ in range(iterations):\n",
    "            if curr.data != curr_reversed.data:\n",
    "                return False\n",
    "            curr = curr.next\n",
    "            curr_reversed = curr_reversed.next\n",
    "        return True"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Overwriting test_palindrome.py\n"
     ]
    }
   ],
   "source": [
    "%%writefile test_palindrome.py\n",
    "from nose.tools import assert_equal\n",
    "\n",
    "\n",
    "class TestPalindrome(object):\n",
    "\n",
    "    def test_palindrome(self):\n",
    "        print('Test: Empty list')\n",
    "        linked_list = MyLinkedList()\n",
    "        assert_equal(linked_list.is_palindrome(), False)\n",
    "\n",
    "        print('Test: Single element list')\n",
    "        head = Node(1)\n",
    "        linked_list = MyLinkedList(head)\n",
    "        assert_equal(linked_list.is_palindrome(), False)\n",
    "\n",
    "        print('Test: Two element list, not a palindrome')\n",
    "        linked_list.append(2)\n",
    "        assert_equal(linked_list.is_palindrome(), False)\n",
    "\n",
    "        print('Test: General case: Palindrome with even length')\n",
    "        head = Node('a')\n",
    "        linked_list = MyLinkedList(head)\n",
    "        linked_list.append('b')\n",
    "        linked_list.append('b')\n",
    "        linked_list.append('a')\n",
    "        assert_equal(linked_list.is_palindrome(), True)\n",
    "\n",
    "        print('Test: General case: Palindrome with odd length')\n",
    "        head = Node(1)\n",
    "        linked_list = MyLinkedList(head)\n",
    "        linked_list.append(2)\n",
    "        linked_list.append(3)\n",
    "        linked_list.append(2)\n",
    "        linked_list.append(1)\n",
    "        assert_equal(linked_list.is_palindrome(), True)\n",
    "\n",
    "        print('Success: test_palindrome')\n",
    "\n",
    "\n",
    "def main():\n",
    "    test = TestPalindrome()\n",
    "    test.test_palindrome()\n",
    "\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    main()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Test: Empty list\n",
      "Test: Single element list\n",
      "Test: Two element list, not a palindrome\n",
      "Test: General case: Palindrome with even length\n",
      "Test: General case: Palindrome with odd length\n",
      "Success: test_palindrome\n"
     ]
    }
   ],
   "source": [
    "%run -i test_palindrome.py"
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.5.0"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 0
}