{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes)." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Problem: Remove duplicates from a linked list\n", "\n", "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm: Hash Map Lookup](#Algorithm:-Hash-Map-Lookup)\n", "* [Code: Hash Map Lookup](#Code:-Hash-Map-Lookup)\n", "* [Algorithm: In-Place](#Algorithm:-In-Place)\n", "* [Code: In-Place](#Code:-In-Place)\n", "* [Unit Test](#Unit-Test)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints and Assumptions\n", "\n", "*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n", "\n", "* Is this a singly or doubly linked list?\n", " * Singly\n", "* Can you insert NULL values in the list?\n", " * No\n", "* Can you use additional data structures?\n", " * Implement both solutions\n", "* Can we assume we already have a linked list class that can be used for this problem?\n", " * Yes" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* Empty linked list\n", "* One element linked list\n", "* Multiple elements\n", "* No duplicates\n", "* One or more duplicates" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: Hash Map Lookup\n", "\n", "* For each node\n", " * If the node's value is in the hash map\n", " * Delete the node\n", " * Else\n", " * Add node's value to the hash map\n", "\n", "Complexity:\n", "* Time: O(n)\n", "* Space: O(m) where m is the number of values in the hash map\n", "\n", "Note:\n", "* Deletion requires two pointers, one to the previous node and one to the current node" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Hash Map Lookup" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": true }, "outputs": [], "source": [ "%run linked_list.py" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "class MyLinkedList(LinkedList):\n", " def remove_dupes(self):\n", " seen_data = set()\n", " curr = self.head\n", " prev = None\n", " while curr is not None:\n", " if curr.data in seen_data:\n", " prev.next = curr.next\n", " else:\n", " seen_data.add(curr.data)\n", " prev = curr\n", " curr = curr.next" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: In-Place\n", "\n", "* For each node\n", " * Compare node with every other node\n", " * If the node's value is in the hash map\n", " * Delete the node\n", " * Else\n", " * Add node's value to the hash map\n", "\n", "Complexity:\n", "* Time: O(n^2)\n", "* Space: In-place\n", "\n", "Note:\n", "* Deletion requires two pointers, one to the previous node and one to the current node\n", "* We'll need to use a 'runner' to check every other node and compare it to the current node" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: In-Place" ] }, { "cell_type": "code", "execution_count": 3, "metadata": { "collapsed": false }, "outputs": [], "source": [ "class MyLinkedListAlt(LinkedList):\n", " def remove_dupes(self):\n", " curr = self.head\n", " while curr is not None:\n", " runner = curr\n", " while runner.next is not None:\n", " if runner.next.data == curr.data:\n", " runner.next = runner.next.next\n", " else:\n", " runner = runner.next\n", " curr = curr.next" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Unit Test" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand. You generally will not be asked to write a unit test like what is shown below.*" ] }, { "cell_type": "code", "execution_count": 4, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Test: Empty list\n", "Test: One element list\n", "Test: General case, duplicates\n", "Test: General case, no duplicates\n", "Success: test_remove_dupes\n", "\n", "Test: Empty list\n", "Test: One element list\n", "Test: General case, duplicates\n", "Test: General case, no duplicates\n", "Success: test_remove_dupes\n", "\n" ] } ], "source": [ "from nose.tools import assert_equal\n", "\n", "class Test(object):\n", " def test_remove_dupes(self, linked_list):\n", " print('Test: Empty list')\n", " linked_list.remove_dupes()\n", " assert_equal(linked_list.get_all_data(), [])\n", "\n", " print('Test: One element list')\n", " linked_list.insert_to_front(2)\n", " linked_list.remove_dupes()\n", " assert_equal(linked_list.get_all_data(), [2])\n", "\n", " print('Test: General case, duplicates')\n", " linked_list.insert_to_front(1)\n", " linked_list.insert_to_front(3)\n", " linked_list.insert_to_front(1)\n", " linked_list.insert_to_front(1)\n", " linked_list.remove_dupes()\n", " assert_equal(linked_list.get_all_data(), [1, 3, 2])\n", "\n", " print('Test: General case, no duplicates')\n", " linked_list.remove_dupes()\n", " assert_equal(linked_list.get_all_data(), [1, 3, 2])\n", " \n", " print('Success: test_remove_dupes\\n')\n", "\n", "if __name__ == '__main__':\n", " test = Test()\n", " linked_list = MyLinkedList(None)\n", " test.test_remove_dupes(linked_list)\n", " linked_list_alt = MyLinkedListAlt(None)\n", " test.test_remove_dupes(linked_list_alt)" ] } ], "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": "2.7.10" } }, "nbformat": 4, "nbformat_minor": 0 }