Added notebook solving the following: Implement the function void Reverse(char* str).

README.md

@@ -6,6 +6,7 @@ Data structures and algorithms practice problems.
 ## Arrays and Strings
 
 * [Check for unique characters in a string](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/unique_chars.ipynb)
+* [Reverse characters in a string](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/reverse_string.ipynb)
 
 ## License
 

arrays-strings/reverse_string.cpp

@@ -0,0 +1,36 @@
+#include <stdio.h>
+
+void Reverse(char* str) {
+    if (str) {
+        char* i = str;	// first letter
+        char* j = str;	// last letter
+
+        // find the end of the string
+        while (*j) {
+            j++;
+        }
+
+        // don't point to the null terminator
+        j--;
+
+        char tmp;
+
+        // swap chars to reverse the string
+        while (i < j) {
+            tmp = *i;
+            *i++ = *j;
+            *j-- = tmp;
+        }
+    }
+}
+
+int main() {
+    char test0[] = "";
+    char test1[] = "foo";
+    Reverse(NULL);
+    Reverse(test0);
+    Reverse(test1);
+    printf("%s \n", test0);
+    printf("%s \n", test1);
+    return 0;
+}

arrays-strings/reverse_string.ipynb

@@ -0,0 +1,154 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement the function void Reverse(char* str)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "\n",
+    "* Based on the function signature, it seems you have to implement this in C/C++?\n",
+    "    * Yes\n",
+    "* Can you use additional data structures?\n",
+    "    * No"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* NULL input\n",
+    "* \"\" -> \"\"\n",
+    "* \"foo bar\" -> \"rab oof\""
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll want to keep two pointers\n",
+    "* i is a pointer to the first char\n",
+    "* j is a pointer to the last char\n",
+    "\n",
+    "To get a pointer to the last char, we need to loop through all characters, take note of null terminator.\n",
+    "\n",
+    "* while i < j\n",
+    "    * swap i and j\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n).\n",
+    "* Space: In-place.\n",
+    "\n",
+    "Note:\n",
+    "* Instead of using i, you can use str instead, although this might not be as intuitive."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "# %load reverse_string.cpp\n",
+    "#include <stdio.h>\n",
+    "\n",
+    "void Reverse(char* str) {\n",
+    "    if (str) {\n",
+    "        char* i = str;\t// first letter\n",
+    "        char* j = str;\t// last letter\n",
+    "        \n",
+    "        // find the end of the string\n",
+    "        while (*j) {\n",
+    "            j++;\n",
+    "        }\n",
+    "        \n",
+    "        // don't point to the null terminator\n",
+    "        j--;\n",
+    "        \n",
+    "        char tmp;\n",
+    "        \n",
+    "        // swap chars to reverse the string\n",
+    "        while (i < j) {\n",
+    "            tmp = *i;\n",
+    "            *i++ = *j;\n",
+    "            *j-- = tmp;\n",
+    "        }\n",
+    "    }\n",
+    "}\n",
+    "\n",
+    "int main() {\n",
+    "    char test0[] = \"\";\n",
+    "    char test1[] = \"foo\";\n",
+    "    Reverse(NULL);\n",
+    "    Reverse(test0);\n",
+    "    Reverse(test1);\n",
+    "    printf(\"%s \\n\", test0);\n",
+    "    printf(\"%s \\n\", test1);\n",
+    "    return 0;\n",
+    "}"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Pythonic Solution(s)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def reverse_string_alt(string):\n",
+    "    return string[::-1]\n",
+    "\n",
+    "def reverse_string_alt2(string):\n",
+    "    return \"\".join(reversed(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
+}