{ "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 string is a permutation of another string.\n", "\n", "* [Constraints](#Constraints)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm: Compare Sorted Strings](#Algorithm:-Compare-Sorted-Strings)\n", "* [Code: Compare Sorted Strings](#Code:-Compare-Sorted-Strings)\n", "* [Algorithm: Hashmap Lookup](#Algorithm:-Hash-Map-Lookup)\n", "* [Code: Hashmap Lookup](#Code:-Hash-Map-Lookup)\n", "* [Unit Test](#Unit-Test)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints\n", "\n", "* Can we assume the string is ASCII?\n", " * Yes\n", " * Note: Unicode strings could require special handling depending on your language\n", "* Is whitespace important?\n", " * Yes\n", "* Is this case sensitive? 'Nib', 'bin' is not a match?\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", "* One or more None inputs -> False\n", "* One or more empty strings -> False\n", "* 'Nib', 'bin' -> False\n", "* 'act', 'cat' -> True\n", "* 'a ct', 'ca t' -> True\n", "* 'dog', 'doggo' -> False" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: Compare Sorted Strings\n", "\n", "Permutations contain the same strings but in different orders. This approach could be slow for large strings due to sorting.\n", "\n", "* Sort both strings\n", "* If both sorted strings are equal\n", " * return True\n", "* Else\n", " * return False\n", "\n", "Complexity:\n", "* Time: O(n log n) from the sort, in general\n", "* Space: O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Compare Sorted Strings" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [], "source": [ "class Permutations(object):\n", "\n", " def is_permutation(self, str1, str2):\n", " if str1 is None or str2 is None:\n", " return False\n", " return sorted(str1) == sorted(str2)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: Hash Map Lookup\n", "\n", "We'll keep a hash map (dict) to keep track of characters we encounter. \n", "\n", "Steps:\n", "* Scan each character\n", "* For each character in each string:\n", " * If the character does not exist in a hash map, add the character to a hash map\n", " * Else, increment the character's count\n", "* If the hash maps for each string are equal\n", " * Return True\n", "* Else\n", " * Return False\n", "\n", "Notes:\n", "* Since the characters are in ASCII, we could potentially use an array of size 128 (or 256 for extended ASCII), where each array index is equivalent to an ASCII value\n", "* Instead of using two hash maps, you could use one hash map and increment character values based on the first string and decrement based on the second string\n", "* You can short circuit if the lengths of each string are not equal, although len() in Python is generally O(1) unlike other languages like C where getting the length of a string is O(n)\n", "\n", "Complexity:\n", "* Time: O(n)\n", "* Space: O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Hash Map Lookup" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "from collections import defaultdict\n", "\n", "\n", "class PermutationsAlt(object):\n", "\n", " def is_permutation(self, str1, str2):\n", " if str1 is None or str2 is None:\n", " return False\n", " if len(str1) != len(str2):\n", " return False\n", " unique_counts1 = defaultdict(int)\n", " unique_counts2 = defaultdict(int)\n", " for char in str1:\n", " unique_counts1[char] += 1\n", " for char in str2:\n", " unique_counts2[char] += 1\n", " return unique_counts1 == unique_counts2" ] }, { "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_permutation_solution.py\n" ] } ], "source": [ "%%writefile test_permutation_solution.py\n", "from nose.tools import assert_equal\n", "\n", "\n", "class TestPermutation(object):\n", "\n", " def test_permutation(self, func):\n", " assert_equal(func(None, 'foo'), False)\n", " assert_equal(func('', 'foo'), False)\n", " assert_equal(func('Nib', 'bin'), False)\n", " assert_equal(func('act', 'cat'), True)\n", " assert_equal(func('a ct', 'ca t'), True)\n", " assert_equal(func('dog', 'doggo'), False)\n", " print('Success: test_permutation')\n", "\n", "\n", "def main():\n", " test = TestPermutation()\n", " permutations = Permutations()\n", " test.test_permutation(permutations.is_permutation)\n", " try:\n", " permutations_alt = PermutationsAlt()\n", " test.test_permutation(permutations_alt.is_permutation)\n", " except NameError:\n", " # Alternate solutions are only defined\n", " # in the solutions file\n", " pass\n", "\n", "\n", "if __name__ == '__main__':\n", " main()" ] }, { "cell_type": "code", "execution_count": 4, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Success: test_permutation\n", "Success: test_permutation\n" ] } ], "source": [ "run -i test_permutation_solution.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 }