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https://github.com/donnemartin/interactive-coding-challenges
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Appended _solution to current notebooks to prepare for challenge and solution notebooks.
This commit is contained in:
Donne Martin
274
arrays_strings/reverse_string/reverse_string_solution.ipynb
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274
arrays_strings/reverse_string/reverse_string_solution.ipynb
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<small><i>This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes).</i></small>"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Problem: Implement a function to reverse a string.\n",
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"\n",
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"* [Constraints](#Constraints)\n",
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"* [Test Cases](#Test-Cases)\n",
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"* [Algorithm](#Algorithm)\n",
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"* [Code](#Code)\n",
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"* [Pythonic-Code](#Pythonic-Code)\n",
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"* [Unit Test](#Unit-Test)\n",
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"* [C Algorithm](C-Algorithm)\n",
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"* [C Code]()"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Constraints\n",
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"\n",
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"*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
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"\n",
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"* Can I assume the string is ASCII?\n",
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" * Yes\n",
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" * Note: Unicode strings could require special handling depending on your language\n",
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"* Can I use the slice operator or the reversed function?\n",
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" * No\n",
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"* Since Python string are immutable, can I use a list instead?\n",
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" * Yes"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Test Cases\n",
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"\n",
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"* NULL input\n",
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"* '' -> ''\n",
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"* 'foo bar' -> 'rab oof'"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Algorithm\n",
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"\n",
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"* Convert the string to a list\n",
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"* Iterate len(string)/2 times, starting with i = 0:\n",
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" * Swap i and len(string) - 1 - i\n",
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" * Sncrement i\n",
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"* Convert the list to a string and return it\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\n",
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"* Space: O(n), additional space converting to/from a list"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Code"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 1,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"def reverse_string(string):\n",
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" if string is None:\n",
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" return None\n",
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" string_list = list(string)\n",
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" string_length = len(string_list)\n",
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" for i in xrange(string_length/2):\n",
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" string_list[i], string_list[string_length-1-i] = \\\n",
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" string_list[string_length-1-i], string_list[i]\n",
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" return ''.join(string_list)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Pythonic-Code\n",
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"\n",
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"This question has an artificial constraint that prevented the use of the slice operator and the reversed method. For completeness, the solutions for these are provided below."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 2,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"def reverse_string_alt(string):\n",
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" if string is None:\n",
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" return None\n",
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" return string[::-1]\n",
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"\n",
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"def reverse_string_alt2(string):\n",
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" if string is None:\n",
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" return None \n",
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" return ''.join(reversed(string))"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Unit Test"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"*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.*"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 3,
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"metadata": {
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"collapsed": false
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},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"Success: test_reversed\n",
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"Success: test_reversed\n",
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"Success: test_reversed\n"
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]
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}
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],
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"source": [
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"from nose.tools import assert_equal\n",
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"\n",
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"class Test(object):\n",
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" def test_reversed(self, func):\n",
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" assert_equal(func(None), None)\n",
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" assert_equal(func(''), '')\n",
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" assert_equal(func('foo bar'), 'rab oof')\n",
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" print('Success: test_reversed')\n",
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"\n",
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"if __name__ == '__main__':\n",
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" test = Test()\n",
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" test.test_reversed(reverse_string)\n",
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" test.test_reversed(reverse_string_alt)\n",
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" test.test_reversed(reverse_string_alt2)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## C Algorithm\n",
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"\n",
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"This is a classic problem in C/C++\n",
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"\n",
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"We'll want to keep two pointers:\n",
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"* i is a pointer to the first char\n",
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"* j is a pointer to the last char\n",
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"\n",
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"To get a pointer to the last char, we need to loop through all characters, take note of null terminator.\n",
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"\n",
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"* while i < j\n",
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" * swap i and j\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\n",
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"* Space: In-place\n",
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"\n",
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"Note:\n",
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"* Instead of using i, you can use str instead, although this might not be as intuitive."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## C Code"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"# %load reverse_string.cpp\n",
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"#include <stdio.h>\n",
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"\n",
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"void Reverse(char* str) {\n",
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" if (str) {\n",
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" char* i = str;\t// first letter\n",
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" char* j = str;\t// last letter\n",
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" \n",
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" // find the end of the string\n",
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" while (*j) {\n",
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" j++;\n",
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" }\n",
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" \n",
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" // don't point to the null terminator\n",
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" j--;\n",
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" \n",
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" char tmp;\n",
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" \n",
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" // swap chars to reverse the string\n",
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" while (i < j) {\n",
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" tmp = *i;\n",
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" *i++ = *j;\n",
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" *j-- = tmp;\n",
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" }\n",
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" }\n",
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"}\n",
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"\n",
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"int main() {\n",
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" char test0[] = \"\";\n",
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" char test1[] = \"foo\";\n",
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" Reverse(NULL);\n",
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" Reverse(test0);\n",
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" Reverse(test1);\n",
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" printf(\"%s \\n\", test0);\n",
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" printf(\"%s \\n\", test1);\n",
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" return 0;\n",
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"}"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 2",
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"language": "python",
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"name": "python2"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 2
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython2",
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"version": "2.7.10"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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