{ "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: Compress a string such that 'AAABCCDDDD' becomes 'A3BCCD4'. Only compress the string if it saves space.\n", "\n", "* [Constraints](#Constraints)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm](#Algorithm)\n", "* [Code](#Code)\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", "* Can you use additional data structures? \n", " * Yes\n", "* Is this case sensitive?\n", " * Yes" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* None -> None\n", "* '' -> ''\n", "* 'AABBCC' -> 'AABBCC'\n", "* 'AAABCCDDDD' -> 'A3BCCD4'" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm\n", "\n", "Since Python strings are immutable, we'll use a list of characters to build the compressed string representation. We'll then convert the list to a string.\n", "\n", "* Calculate the size of the compressed string\n", " * Note the constraint about compressing only if it saves space\n", "* If the compressed string size is >= string size, return string\n", "* Create compressed_string\n", " * For each char in string\n", " * If char is the same as last_char, increment count\n", " * Else\n", " * If the count is more than 2\n", " * Append last_char to compressed_string\n", " * append count to compressed_string\n", " * count = 1\n", " * last_char = char\n", " * If count is 1\n", " * Append last_char to compressed_string\n", " * count = 1\n", " * last_char = char\n", " * If count is 2\n", " * Append last_char to compressed_string\n", " * Append last_char to compressed_string once more\n", " * count = 1\n", " * last_char = char\n", " * Append last_char to compressed_string\n", " * Append count to compressed_string\n", " * Return compressed_string\n", "\n", "Complexity:\n", "* Time: O(n)\n", "* Space: O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code" ] }, { "cell_type": "code", "execution_count": 4, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def compress_string(string):\n", "\tif string is None or len(string) == 0:\n", "\t\treturn string\n", "\n", "\t# Calculate the size of the compressed string\n", "\tsize = 0\n", "\tlast_char = string[0]\n", "\tfor char in string:\n", "\t\tif char != last_char:\n", "\t\t\tsize += 2\n", "\t\t\tlast_char = char\n", "\tsize += 2\n", "\n", "\t# If the compressed string size is greater than\n", "\t# or equal to string size, return original string\n", "\tif size >= len(string):\n", "\t\treturn string\n", "\n", "\t# Create compressed_string\n", "\t# New objective:\n", "\t\t# Single characters are to be left as is\n", "\t\t# Double characters are to be left as are\n", "\tcompressed_string = list()\n", "\tcount = 0\n", "\tlast_char = string[0]\n", "\tfor char in string:\n", "\t\tif char == last_char:\n", "\t\t\tcount += 1\n", "\t\telse:\n", "\t\t\t# Do the old compression tricks only if count exceeds two\n", "\t\t\tif count > 2:\n", "\t\t\t\tcompressed_string.append(last_char)\n", "\t\t\t\tcompressed_string.append(str(count))\n", "\t\t\t\tcount = 1\n", "\t\t\t\tlast_char = char\n", "\t\t\t# If count is either 1 or 2 (or 0?)\n", "\t\t\t# FIXME: CAN COUNT BE 0?\n", "\t\t\telse:\n", "\t\t\t\t# If count is 1, leave the char as is\n", "\t\t\t\tif count == 1:\n", "\t\t\t\t\tcompressed_string.append(last_char)\n", "\t\t\t\t\tcount = 1\n", "\t\t\t\t\tlast_char = char\n", "\t\t\t\t# If count is 2, append the character twice\n", "\t\t\t\telse:\n", "\t\t\t\t\tcompressed_string.append(last_char)\n", "\t\t\t\t\tcompressed_string.append(last_char)\n", "\t\t\t\t\tcount = 1\n", "\t\t\t\t\tlast_char = char\n", "\tcompressed_string.append(last_char)\n", "\tcompressed_string.append(str(count))\n", "\n", "\t# Convert the characters in the list to a string\n", "\treturn \"\".join(compressed_string)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Unit Test" ] }, { "cell_type": "code", "execution_count": 5, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Overwriting test_compress.py\n" ] } ], "source": [ "%%writefile test_compress.py\n", "from nose.tools import assert_equal\n", "\n", "\n", "class TestCompress(object):\n", "\n", " def test_compress(self, func):\n", " assert_equal(func(None), None)\n", " assert_equal(func(''), '')\n", " assert_equal(func('AABBCC'), 'AABBCC')\n", " assert_equal(func('AAABCCDDDD'), 'A3BCCD4')\n", " assert_equal(func('aaBCCEFFFFKKMMMMMMP taaammanlaarrrr seeeeeeeeek tooo'), 'aaBCCEF4KKM6P ta3mmanlaar4 se9k to3')\n", " print('Success: test_compress')\n", "\n", "\n", "def main():\n", " test = TestCompress()\n", " test.test_compress(compress_string)\n", "\n", "\n", "if __name__ == '__main__':\n", " main()" ] }, { "cell_type": "code", "execution_count": 7, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Success: test_compress\n" ] } ], "source": [ "%run -i test_compress.py" ] }, { "cell_type": "code", "execution_count": null, "metadata": { "collapsed": true }, "outputs": [], "source": [] } ], "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": "3.4.3" } }, "nbformat": 4, "nbformat_minor": 0 }