From 69df72a803bf5aa6cc3644290d772d8ff832b12e Mon Sep 17 00:00:00 2001 From: Donne Martin Date: Thu, 7 May 2015 19:56:25 -0400 Subject: [PATCH] Added notebook solving the following: Find the kth to last element of a linked list. --- README.md | 1 + linked-lists/kth-to-last-elem.ipynb | 168 ++++++++++++++++++++++++++++ 2 files changed, 169 insertions(+) create mode 100644 linked-lists/kth-to-last-elem.ipynb diff --git a/README.md b/README.md index d1b44af..31ab1bd 100644 --- a/README.md +++ b/README.md @@ -16,6 +16,7 @@ Continually updated IPython Notebooks containing algorithms and data structures. * [Linked list with insert, find, delete, and print methods](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/linked-list.ipynb) * [Remove duplicates from a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/remove-duplicates.ipynb) +* [Find the kth to last element of a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/kth-to-last-elem.ipynb) ## License diff --git a/linked-lists/kth-to-last-elem.ipynb b/linked-lists/kth-to-last-elem.ipynb new file mode 100644 index 0000000..f064e44 --- /dev/null +++ b/linked-lists/kth-to-last-elem.ipynb @@ -0,0 +1,168 @@ +{ + "cells": [ + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Problem: Find the kth to last element of a linked list\n", + "\n", + "* [Clarifying Questions](#Clarifying-Questions)\n", + "* [Test Cases](#Test-Cases)\n", + "* [Algorithm](#Algorithm)\n", + "* [Code](#Code)" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Clarifying Questions\n", + "\n", + "* Do we assume the linked list class already exists?\n", + " * No, create one\n", + "* Is k an integer?\n", + " * Yes\n", + "* If k = 0, does this return the last element?\n", + " * Yes\n", + "* What happens if k is greater than or equal to the length of the linked list?\n", + " * Return None\n", + "* Can you insert NULL values in the list?\n", + " * No\n", + "* Can use additional data structures?\n", + " * No" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Test Cases\n", + "\n", + "* Empty list\n", + "* k is not an integer\n", + "* k is >= the length of the linked list\n", + "* One element, k = 0\n", + "* General case with many elements, k < length of linked list" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Algorithm\n", + "\n", + "* Check for edge cases above, returning None for errors\n", + "* Setup two pointers, current and previous\n", + "* Give current a headstart, incrementing it once for k = 1, twice for k = 2, etc\n", + "* Increment both pointers until current reaches the end\n", + "* Return the value of previous\n", + "\n", + "Complexity:\n", + "* Time: O(n)\n", + "* Space: In-place" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Code" + ] + }, + { + "cell_type": "code", + "execution_count": null, + "metadata": { + "collapsed": false + }, + "outputs": [], + "source": [ + "class Node(object):\n", + " def __init__(self, data):\n", + " self.data = data\n", + " self.next = None\n", + " \n", + "class LinkedList(object):\n", + " def __init__(self, head):\n", + " self.head = head\n", + " \n", + " def __len__(self):\n", + " curr = self.head\n", + " counter = 0\n", + " while curr is not None:\n", + " counter += 1\n", + " curr = curr.next\n", + " return counter\n", + " \n", + " def kth_to_last_elem(self, k):\n", + " if self.head is None:\n", + " return\n", + " if type(k) != int:\n", + " return\n", + " if k >= len(self):\n", + " return\n", + " curr = self.head\n", + " prev = self.head\n", + " counter = 0\n", + " while counter < k:\n", + " curr = curr.next\n", + " counter += 1\n", + " if curr is None:\n", + " return\n", + " while curr.next is not None:\n", + " curr = curr.next\n", + " prev = prev.next\n", + " return prev.data\n", + " \n", + " def insert_to_front(self, data):\n", + " if data is None:\n", + " return\n", + " node = Node(data)\n", + " if self.head is None:\n", + " self.head = node\n", + " else:\n", + " node.next = self.head\n", + " self.head = node\n", + " \n", + "# Empty list\n", + "linked_list = LinkedList(None)\n", + "print(linked_list.kth_to_last_elem(0))\n", + "# k is not an integer\n", + "print(linked_list.kth_to_last_elem('a'))\n", + "# k is >= the length of the linked list\n", + "print(linked_list.kth_to_last_elem(100))\n", + "# One element, k = 0\n", + "head = Node(2)\n", + "linked_list = LinkedList(head)\n", + "print(linked_list.kth_to_last_elem(0))\n", + "# General case with many elements, k < length of linked list\n", + "linked_list.insert_to_front(1)\n", + "linked_list.insert_to_front(3)\n", + "linked_list.insert_to_front(5)\n", + "linked_list.insert_to_front(7)\n", + "print(linked_list.kth_to_last_elem(2))" + ] + } + ], + "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 +}