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https://github.com/donnemartin/interactive-coding-challenges
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Renamed top-level folders to use underscores instead of dashes.
This commit is contained in:
Donne Martin
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arrays_strings/unique_chars/__init__.py
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arrays_strings/unique_chars/__init__.py
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arrays_strings/unique_chars/unique_chars.ipynb
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arrays_strings/unique_chars/unique_chars.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 an algorithm to determine if a string has all unique characters\n",
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"\n",
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"* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
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"* [Test Cases](#Test-Cases)\n",
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"* [Algorithm 1: Sets and Length Comparison](#Algorithm-1:-Sets-and-Length-Comparison)\n",
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"* [Code: Sets and Length Comparison](#Code:-Sets-and-Length-Comparison)\n",
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"* [Algorithm 2: Hash Map Lookup](#Algorithm-2:-Hash-Map-Lookup)\n",
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"* [Code: Hash Map Lookup](#Code:-Hash-Map-Lookup)\n",
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"* [Algorithm 3: In-Place](#Algorithm-3:-In-Place)\n",
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"* [Code: In-Place](#Code:-In-Place)\n",
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"* [Unit Test](#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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"## Constraints and Assumptions\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 you use additional data structures? \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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"* '' -> True\n",
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"* 'foo' -> False\n",
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"* 'bar' -> True"
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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 1: Sets and Length Comparison\n",
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"\n",
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"A set is an unordered collection of unique elements. \n",
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"\n",
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"* If the length of the set(string) equals the length of the string\n",
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" * Return True\n",
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"* Else\n",
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" * Return False\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: Additional O(m), where m is the number of unique characters in the set"
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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: Sets and Length Comparison"
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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 unique_chars(string):\n",
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" return len(set(string)) == len(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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"## Algorithm 2: Hash Map Lookup\n",
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"\n",
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"We'll keep a hash map (set) to keep track of unique characters we encounter. \n",
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"\n",
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"Steps:\n",
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"* Scan each character\n",
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"* For each character:\n",
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" * If the character does not exist in a hash map, add the character to a hash map\n",
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" * Else, return False\n",
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"* Return True\n",
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"\n",
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"Notes:\n",
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"* We could also use a dictionary, but it seems more logical to use a set as it does not contain duplicate elements\n",
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"* Since the characters are in ASCII, we could potentially use an array of size 128 (or 256 for extended ASCII)\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: Additional O(m), where m is the number of unique characters in the hash map"
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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: Hash Map Lookup"
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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 unique_chars_hash(string):\n",
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" chars_set = set()\n",
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" for char in string:\n",
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" if char in chars_set:\n",
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" return False\n",
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" else:\n",
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" chars_set.add(char)\n",
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" return True"
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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 3: In-Place\n",
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"\n",
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"Assume we cannot use additional data structures, which will eliminate the fast lookup O(1) time provided by our hash map. \n",
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"* Scan each character\n",
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"* For each character:\n",
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" * Scan all [other] characters in the array\n",
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" * Exluding the current character from the scan is rather tricky in Python and results in a non-Pythonic solution\n",
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" * If there is a match, return False\n",
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"* Return True\n",
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"\n",
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"Algorithm Complexity:\n",
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"* Time: O(n^2)\n",
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"* Space: In-place"
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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: In-Place"
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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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"source": [
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"def unique_chars_inplace(string):\n",
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" for char in string:\n",
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" if string.count(char) > 1:\n",
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" return False\n",
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" return True"
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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": 4,
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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_unique_chars\n",
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"Success: test_unique_chars\n",
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"Success: test_unique_chars\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_unique_chars(self, func):\n",
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" assert_equal(func(''), True)\n",
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" assert_equal(func('foo'), False)\n",
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" assert_equal(func('bar'), True)\n",
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" print('Success: test_unique_chars')\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_unique_chars(unique_chars)\n",
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" test.test_unique_chars(unique_chars_hash)\n",
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" test.test_unique_chars(unique_chars_inplace)"
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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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