{ "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: Implement an algorithm to determine if a string has all unique characters\n", "\n", "* [Constraints](#Constraints)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm 1: Sets and Length Comparison](#Algorithm-1:-Sets-and-Length-Comparison)\n", "* [Code: Sets and Length Comparison](#Code:-Sets-and-Length-Comparison)\n", "* [Algorithm 2: Hash Map Lookup](#Algorithm-2:-Hash-Map-Lookup)\n", "* [Code: Hash Map Lookup](#Code:-Hash-Map-Lookup)\n", "* [Algorithm 3: In-Place](#Algorithm-3:-In-Place)\n", "* [Code: In-Place](#Code:-In-Place)\n", "* [Unit Test](#Unit-Test)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints\n", "\n", "* Can you assume the string is ASCII?\n", " * Yes\n", " * Note: Unicode strings could require special handling depending on your language\n", "* Can we assume this is case sensitive?\n", " * Yes\n", "* Can you use additional data structures? \n", " * Yes" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* '' -> True\n", "* 'foo' -> False\n", "* 'bar' -> True" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm 1: Sets and Length Comparison\n", "\n", "A set is an unordered collection of unique elements. \n", "\n", "* If the length of the set(string) equals the length of the string\n", " * Return True\n", "* Else\n", " * Return False\n", " \n", "Complexity:\n", "* Time: O(n)\n", "* Space: Additional O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Sets and Length Comparison" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def unique_chars(string):\n", " return len(set(string)) == len(string)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm 2: Hash Map Lookup\n", "\n", "We'll keep a hash map (set) to keep track of unique characters we encounter. \n", "\n", "Steps:\n", "* Scan each character\n", "* For each character:\n", " * If the character does not exist in a hash map, add the character to a hash map\n", " * Else, return False\n", "* Return True\n", "\n", "Notes:\n", "* We could also use a dictionary, but it seems more logical to use a set as it does not contain duplicate elements\n", "* Since the characters are in ASCII, we could potentially use an array of size 128 (or 256 for extended ASCII)\n", "\n", "Complexity:\n", "* Time: O(n)\n", "* Space: Additional O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Hash Map Lookup" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def unique_chars_hash(string):\n", " chars_set = set()\n", " for char in string:\n", " if char in chars_set:\n", " return False\n", " else:\n", " chars_set.add(char)\n", " return True" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm 3: In-Place\n", "\n", "Assume we cannot use additional data structures, which will eliminate the fast lookup O(1) time provided by our hash map. \n", "* Scan each character\n", "* For each character:\n", " * Scan all [other] characters in the array\n", " * Exluding the current character from the scan is rather tricky in Python and results in a non-Pythonic solution\n", " * If there is a match, return False\n", "* Return True\n", "\n", "Algorithm Complexity:\n", "* Time: O(n^2)\n", "* Space: O(1)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: In-Place" ] }, { "cell_type": "code", "execution_count": 3, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def unique_chars_inplace(string):\n", " for char in string:\n", " if string.count(char) > 1:\n", " return False\n", " return True" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Unit Test" ] }, { "cell_type": "code", "execution_count": 4, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Overwriting test_unique_chars.py\n" ] } ], "source": [ "%%writefile test_unique_chars.py\n", "from nose.tools import assert_equal\n", "\n", "\n", "class TestUniqueChars(object):\n", "\n", " def test_unique_chars(self, func):\n", " assert_equal(func(''), True)\n", " assert_equal(func('foo'), False)\n", " assert_equal(func('bar'), True)\n", " print('Success: test_unique_chars')\n", "\n", "def main():\n", " test = TestUniqueChars()\n", " test.test_unique_chars(unique_chars)\n", " try:\n", " test.test_unique_chars(unique_chars_hash)\n", " test.test_unique_chars(unique_chars_inplace)\n", " except NameError:\n", " # Alternate solutions are only defined\n", " # in the solutions file\n", " pass\n", " \n", "if __name__ == '__main__':\n", " main()" ] }, { "cell_type": "code", "execution_count": 5, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Success: test_unique_chars\n", "Success: test_unique_chars\n", "Success: test_unique_chars\n" ] } ], "source": [ "%run -i test_unique_chars.py" ] } ], "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 }