From b6f3e573780a0e15397c5ac2176860a8cf9ba853 Mon Sep 17 00:00:00 2001 From: Donne Martin Date: Tue, 19 May 2015 17:20:56 -0400 Subject: [PATCH] Added notebook solving the following: Determine if a linked list is a palindrome. --- README.md | 2 + linked-lists/palindrome.ipynb | 166 ++++++++++++++++++++++++++++++++++ 2 files changed, 168 insertions(+) create mode 100644 linked-lists/palindrome.ipynb diff --git a/README.md b/README.md index 7c9874a..6431ea8 100644 --- a/README.md +++ b/README.md @@ -28,6 +28,8 @@ Continually updated IPython Notebooks containing coding problems and solutions ( * [Partition a linked list around a given value](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/partition.ipynb) * [Add two numbers whose digits are stored in a linked list in reverse order](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/add-reverse.ipynb) * [Find the start of a linked list loop](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/find-loop-start.ipynb) +* [Determine if a linked list is a palindrome](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/palindrome.ipynb) + ## Stacks and Queues diff --git a/linked-lists/palindrome.ipynb b/linked-lists/palindrome.ipynb new file mode 100644 index 0000000..3b27e25 --- /dev/null +++ b/linked-lists/palindrome.ipynb @@ -0,0 +1,166 @@ +{ + "cells": [ + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Problem: Determine if a linked list is a palindrome.\n", + "\n", + "* [Clarifying Questions](#Clarifying-Questions)\n", + "* [Test Cases](#Test-Cases)\n", + "* [Algorithm](#Algorithm)\n", + "* [Code](#Code)\n", + "* [Pythonic-Code](#Pythonic-Code)" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Clarifying Questions\n", + "\n", + "* Is a single character or number a palindrome?\n", + " * No" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Test Cases\n", + "\n", + "* Empty list\n", + "* Single element list\n", + "* Two element list, not a palindrome\n", + "* Three element list, not a palindrome\n", + "* General case: Palindrome with even length\n", + "* General case: Palindrome with odd length" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Algorithm\n", + "\n", + "* Reverse the 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(1)\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": null, + "metadata": { + "collapsed": true + }, + "outputs": [], + "source": [ + "%run linked_list.py" + ] + }, + { + "cell_type": "code", + "execution_count": null, + "metadata": { + "collapsed": false + }, + "outputs": [], + "source": [ + "class MyLinkedList(LinkedList):\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", + " while curr is not None:\n", + " reversed_list.insert_to_front(curr.data)\n", + " length += 1\n", + " curr = curr.next\n", + " iterations_to_compare_half = length / 2\n", + " curr = self.head\n", + " curr_reversed = reversed_list.head\n", + " for _ in xrange(0, iterations_to_compare_half):\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": "code", + "execution_count": null, + "metadata": { + "collapsed": false + }, + "outputs": [], + "source": [ + "print('Empty list')\n", + "linked_list = MyLinkedList()\n", + "print(linked_list.is_palindrome())\n", + "print('Single element list')\n", + "head = Node(1)\n", + "linked_list = MyLinkedList(head)\n", + "print(linked_list.is_palindrome())\n", + "print('Two element list, not a palindrome')\n", + "linked_list.append(2)\n", + "print(linked_list.is_palindrome())\n", + "print('Three element list, not a palindrome')\n", + "linked_list.append(3)\n", + "print(linked_list.is_palindrome())\n", + "print('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", + "print(linked_list.is_palindrome())\n", + "print('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", + "print(linked_list.is_palindrome())" + ] + } + ], + "metadata": { + "kernelspec": { + "display_name": "Python 2", + "language": "python", + "name": "python2" + }, + "language_info": { + "codemirror_mode": { + "name": "ipython", + "version": 2 + }, + "file_extension": ".py", + "mimetype": "text/x-python", + "name": "python", + "nbconvert_exporter": "python", + "pygments_lexer": "ipython2", + "version": "2.7.9" + } + }, + "nbformat": 4, + "nbformat_minor": 0 +}