Added notebook solving the following: Given a String, Encode In-Place all Spaces.

README.md

@@ -8,6 +8,7 @@ Continually updated IPython Notebooks containing algorithms and data structures.
 * [Check if a string contains unique characters](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/arrays-strings/unique_chars.ipynb)
 * [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)
 
 ## License
 

arrays-strings/replace_char.ipynb

@@ -0,0 +1,132 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Given a String, Replace In-Place all Spaces with '%20'\n",
+    "\n",
+    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "\n",
+    "* Is the string ASCII (extended)?  Or Unicode?\n",
+    "    * ASCII extended, which is 256 characters\n",
+    "* Is there enough space in the data structure for this operation?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* NULL\n",
+    "* ' ' -> '%20'\n",
+    "* ' foo bar ' -> '%20foo%20bar%20'\n",
+    "* 'foo' -> 'foo'"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/replace_string.jpg)\n",
+    "\n",
+    "Since Python strings are immutable, we'll use a bytearray instead to exercise in-place string manipulation as you would get with a C string (which is null terminated, as seen in the above diagram).  For the python bytearray we will not use a null terminator.\n",
+    "\n",
+    "* Count the number of spaces in the bytearray\n",
+    "* Determine the new bytearray length\n",
+    "* For each character code in the bytearray, starting from the end of the string\n",
+    "    * If the character code is a space\n",
+    "        * bytearray[new length] = '0',\n",
+    "        * bytearray[new length - 1] = '2',\n",
+    "        * bytearray[new length - 2] = '%',\n",
+    "        * new length -= 3\n",
+    "    * Else\n",
+    "        * bytearray[new length] = character code,\n",
+    "        * new length -= 1\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: In-place"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def encode_spaces(string, length):\n",
+    "    if string is None:\n",
+    "        return\n",
+    "    num_spaces = string.count(' ')\n",
+    "    new_length = length + 2 * num_spaces - 1\n",
+    "    for i in xrange(length-1, -1, -1):\n",
+    "        if chr(string[i]) == ' ':\n",
+    "            string[new_length] = '0'\n",
+    "            string[new_length - 1] = '2'\n",
+    "            string[new_length - 2] = '%'\n",
+    "            new_length -= 3\n",
+    "        else:\n",
+    "            string[new_length] = string[i]\n",
+    "            new_length -= 1\n",
+    "\n",
+    "str0 = None\n",
+    "str1 = bytearray(' ||')\n",
+    "str2 = bytearray(' foo bar ||||||')\n",
+    "str3 = bytearray('foo')\n",
+    "encode_spaces(str0, 0)\n",
+    "encode_spaces(str1, 1)\n",
+    "encode_spaces(str2, 9)\n",
+    "encode_spaces(str3, 3)\n",
+    "print(str0)\n",
+    "print(str1)\n",
+    "print(str2)\n",
+    "print(str3)"
+   ]
+  }
+ ],
+ "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
+}