Added notebook solving the following: Implement an algorithm to determine if a string has all unique characters.

arrays-strings/unique_chars.ipynb

@@ -0,0 +1,167 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement an algorithm to determine if a string has all unique characters"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "* Is the string in ASCII (extended?) or Unicode?  \n",
+    "    * ASCII extended, which is 256 characters.\n",
+    "* Can you use additional data structures?  \n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* \"\" -> True\n",
+    "* \"foo\" -> False\n",
+    "* \"bar\" -> True"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll keep a hash map (set) to keep track of unique characters we encounter.  \n",
+    "\n",
+    "Note:\n",
+    "* We could also use a dictionary, but it seems more logical to use a set as it does not contain duplicate elements.\n",
+    "* Since the characters are in ASCII, we could potentially use an array of size 128 (or 256 for extended ASCII)\n",
+    "\n",
+    "Steps:\n",
+    "* Scan each character.\n",
+    "* For each character:\n",
+    "    * If the character does not exist in a hash map, add the character to a hash map.\n",
+    "    * Else, return False.\n",
+    "* Return True\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n).\n",
+    "* Space: Additional O(m), where m is the number of unique characters in the hash map."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def unique_chars(string):\n",
+    "    chars_set = set()\n",
+    "    for char in string:\n",
+    "        if char in chars_set:\n",
+    "            return False\n",
+    "        else:\n",
+    "            chars_set.add(char)\n",
+    "    return True\n",
+    "\n",
+    "print(unique_chars(''))\n",
+    "print(unique_chars('foo'))\n",
+    "print(unique_chars('bar'))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm: No Additional Data Structures\n",
+    "\n",
+    "Since we cannot use additional data structures, this will eliminate the fast lookup O(1) time provided by our hash map.\n",
+    "* Scan each character.\n",
+    "* For each character:\n",
+    "    * Scan all [other] characters in the array\n",
+    "        * Exluding the current character from the scan is rather tricky in Python and results in a non-Pythonic solution\n",
+    "        * If there is a match, return False\n",
+    "* Return True\n",
+    "\n",
+    "Algorithm Complexity:\n",
+    "* Time: O(n^2).\n",
+    "* Space: In-place."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def unique_chars_alt(string):\n",
+    "    for char in string:\n",
+    "        if string.count(char) > 1:\n",
+    "            return False\n",
+    "    return True"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Pythonic Solution(s)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def unique_chars_alt2(string):\n",
+    "    return len(set(string)) == len(string)"
+   ]
+  }
+ ],
+ "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": "2.7.9"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}