{ "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 'A3B1C2D4'\n", "\n", "* [Constraints](#Constraints)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm: List](#Algorithm:-List)\n", "* [Code: List](#Code:-List)\n", "* [Algorithm: Byte Array](#Algorithm:-Byte-Array)\n", "* [Code: Byte array](#Code:-Byte-Array)\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\n", "* Do you compress even if it doesn't save space?\n", " * No" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* None -> None\n", "* '' -> ''\n", "* 'AABBCC' -> 'AABBCC'\n", "* 'AAABCCDDDD' -> 'A3B1C2D4'" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: List\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", " * Append last_char to compressed_string\n", " * append count to compressed_string\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: List" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def compress_string(string):\n", " if string is None or len(string) == 0:\n", " return string\n", " \n", " # Calculate the size of the compressed string\n", " size = 0\n", " last_char = string[0]\n", " for char in string:\n", " if char != last_char:\n", " size += 2\n", " last_char = char\n", " size += 2\n", " \n", " # If the compressed string size is greater than \n", " # or equal to string size, return string\n", " if size >= len(string):\n", " return string\n", "\n", " # Create compressed_string\n", " compressed_string = list()\n", " count = 0\n", " last_char = string[0]\n", " for char in string:\n", " if char == last_char:\n", " count += 1\n", " else:\n", " compressed_string.append(last_char)\n", " compressed_string.append(str(count))\n", " count = 1\n", " last_char = char\n", " compressed_string.append(last_char)\n", " compressed_string.append(str(count))\n", " \n", " # Convert the characters in the list to a string\n", " return \"\".join(compressed_string)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm: Byte Array\n", "\n", "As a bonus solution, we can also solve this problem with a byte array.\n", "\n", "The byte array algorithm similar when using a list, except we will need to work with the bytearray's character codes (using the function ord) instead of the characters as when we implemented this solution with a list.\n", "\n", "Complexity:\n", "* Time: O(n)\n", "* Space: O(n)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code: Byte Array" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "def compress_string_alt(string):\n", " if string is None or len(string) == 0:\n", " return string\n", " \n", " # Calculate the size of the compressed string\n", " size = 0\n", " last_char_code = string[0]\n", " for char_code in string:\n", " if char_code != last_char_code:\n", " size += 2\n", " last_char_code = char_code\n", " size += 2\n", " \n", " # If the compressed string size is greater than \n", " # or equal to string size, return string \n", " if size >= len(string):\n", " return string\n", " \n", " # Create compressed_string\n", " compressed_string = bytearray(size)\n", " pos = 0\n", " count = 0\n", " last_char_code = string[0]\n", " for char_code in string:\n", " if char_code == last_char_code:\n", " count += 1\n", " else:\n", " compressed_string[pos] = last_char_code\n", " compressed_string[pos+1] = ord(str(count))\n", " pos += 2\n", " count = 1\n", " last_char_code = char_code\n", " compressed_string[pos] = last_char_code\n", " compressed_string[pos+1] = ord(str(count))\n", " return str(compressed_string)" ] }, { "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_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'), 'A3B1C2D4')\n", " print('Success: test_compress')\n", "\n", "def main():\n", " test = TestCompress()\n", " test.test_compress(compress_string)\n", " try:\n", " test.test_compress(compress_string_alt)\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": 4, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Success: test_compress\n", "Success: test_compress\n" ] } ], "source": [ "%run -i test_compress.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 }