Added notebook solving the following: Compress a string such that aabbb becomes a2b3.

README.md

@@ -9,6 +9,7 @@ Continually updated IPython Notebooks containing algorithms and data structures.
 * [Reverse characters in a string](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/arrays-strings/reverse_string.ipynb)
 * [Check if a string is a permutation of another](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/arrays-strings/permutation.ipynb)
 * [Encode spaces in a string in-place](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/arrays-strings/replace_char.ipynb)
+* [Compress a string](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/arrays-strings/compress.ipynb)
 
 ## License
 

arrays-strings/compress.ipynb

@@ -0,0 +1,263 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Compress a String Such that 'AAABCCDDDD' Becomes 'A3B1C2D4'.  Only Compress if it Saves Space.\n",
+    "\n",
+    "* [Clarifying Questions](#Clarifying-Questions)\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)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "\n",
+    "* Is the string ASCII (extended)?  Or Unicode?\n",
+    "    * ASCII extended, which is 256 characters\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",
+    "* NULL\n",
+    "* '' -> ''\n",
+    "* 'ABC' -> 'ABC'\n",
+    "* 'AAABCCDDDD' -> 'A3B1C2D4'"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm: List\n",
+    "\n",
+    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/compress_string.jpg)\n",
+    "\n",
+    "Since Python strings are immutable, we'll use a list to exercise array manipulation.  Note using a list vs a bytearray will will result in additional space to create the list and to convert the list to a string.\n",
+    "\n",
+    "* If string is empty return string\n",
+    "* count = 0\n",
+    "* size = 0\n",
+    "* last_char = first char in string\n",
+    "* For each char in string\n",
+    "    * If char == last_char\n",
+    "        count++\n",
+    "    * Else\n",
+    "        size += 2\n",
+    "        count++\n",
+    "        last_char = char\n",
+    "* size += 2\n",
+    "* If the compressed string size is >= string size, return string\n",
+    "* Create compressed_string\n",
+    "* For each char in string\n",
+    "    * If char == last_char\n",
+    "        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(m) where m is the size of the compressed bytearray"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code: List"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def compress_string(string):\n",
+    "    if string is None or len(string) == 0:\n",
+    "        return string\n",
+    "    size = 0\n",
+    "    count = 0\n",
+    "    last_char = string[0]\n",
+    "    for char in string:\n",
+    "        if char == last_char:\n",
+    "            count += 1\n",
+    "        else:\n",
+    "            size += 2\n",
+    "            count = 1\n",
+    "            last_char = char\n",
+    "    size += 2\n",
+    "    if size >= len(string):\n",
+    "        return 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",
+    "    return \"\".join(compressed_string)\n",
+    "\n",
+    "string0 = None\n",
+    "string1 = ''\n",
+    "string2 = 'ABC'\n",
+    "string3 = 'AAABCCDDDD'\n",
+    "print(compress_string(string0))\n",
+    "print(compress_string(string1))\n",
+    "print(compress_string(string2))\n",
+    "print(compress_string(string3))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm: Byte Array\n",
+    "\n",
+    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/compress_string.jpg)\n",
+    "\n",
+    "Since Python strings are immutable, we'll use a bytearray to exercise array manipulation.  We could use a list of characters to create the compressed string then convert it to a string in the end, but this will result in additional space.\n",
+    "\n",
+    "* If bytearray is empty return bytearray\n",
+    "* count = 0\n",
+    "* size = 0\n",
+    "* last_char_code = first char code in bytearray\n",
+    "* For each char code in bytearray\n",
+    "    * If char code == last_char_code\n",
+    "        count++\n",
+    "    * Else\n",
+    "        size += 2\n",
+    "        count++\n",
+    "        last_char_code = char code\n",
+    "* size += 2\n",
+    "* If the compressed bytearray size is >= bytearray size, return string\n",
+    "* Create compressed_bytearray\n",
+    "* pos = 0\n",
+    "* For each char code in bytearray\n",
+    "    * If char code == last_char_code\n",
+    "        count++\n",
+    "    * Else\n",
+    "        * compressed_bytearray[pos] = last_char_code\n",
+    "        * compressed_bytearray[pos + 1] = count\n",
+    "        * pos += 2\n",
+    "        * count = 1\n",
+    "        * last_char_code = char code\n",
+    "    * compressed_bytearray[pos] = last_char_code\n",
+    "    * compressed_bytearray[pos + 1] = count\n",
+    "* return compressed_bytearray\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(m) where m is the size of the compressed bytearray"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code: Byte Array"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def compress_string(string):\n",
+    "    if string is None or len(string) == 0:\n",
+    "        return string\n",
+    "    size = 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",
+    "            size += 2\n",
+    "            count = 1\n",
+    "            last_char_code = char_code\n",
+    "    size += 2\n",
+    "    if size >= len(string):\n",
+    "        return 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 compressed_string\n",
+    "\n",
+    "string0 = None\n",
+    "string1 = bytearray('')\n",
+    "string2 = bytearray('ABC')\n",
+    "string3 = bytearray('AAABCCDDDD')\n",
+    "print(compress_string(string0))\n",
+    "print(compress_string(string1))\n",
+    "print(compress_string(string2))\n",
+    "print(compress_string(string3))"
+   ]
+  }
+ ],
+ "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"
+  }
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+ "nbformat_minor": 0
+}