Added python solution to the following: Implement a function to reverse a string.

arrays-strings/reverse_string.ipynb

@@ -11,13 +11,15 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Problem: Implement the function void Reverse(char* str)\n",
+    "## Problem: Implement a function to reverse a string.\n",
     "\n",
     "* [Clarifying Questions](#Clarifying-Questions)\n",
     "* [Test Cases](#Test-Cases)\n",
     "* [Algorithm](#Algorithm)\n",
     "* [Code](#Code)\n",
-    "* [Pythonic-Code: Not In-Place](#Pythonic-Code:-Not-In-Place)"
+    "* [C Algorithm](C-Algorithm)\n",
+    "* [C Code]()\n",
+    "* [Pythonic-Code](#Pythonic-Code)"
    ]
   },
   {
@@ -26,10 +28,10 @@
    "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, do the operation in-place"
+    "* Can I use the slice operator or the reversed function?\n",
+    "    * No\n",
+    "* Since Python string are immutable, can I use a list instead?\n",
+    "    * Yes"
    ]
   },
   {
@@ -43,12 +45,80 @@
     "* 'foo bar' -> 'rab oof'"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "from nose.tools import assert_equal\n",
+    "\n",
+    "class Test(object):\n",
+    "    def test_reversed(self, func):\n",
+    "        assert_equal(func(None), None)\n",
+    "        assert_equal(func(''), '')\n",
+    "        assert_equal(func('foo bar'), 'rab oof')\n",
+    "\n",
+    "def run_tests(func):\n",
+    "    test = Test()\n",
+    "    test.test_reversed(func)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
     "## Algorithm\n",
     "\n",
+    "* Convert the string to a list\n",
+    "* Iterate len(string)/2 times, starting with i = 0:\n",
+    "    * Swap i and len(string) - 1 - i\n",
+    "    * Sncrement i\n",
+    "* Convert the list to a string and return it\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n), additional space converting to/from a list"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def reverse_string(string):\n",
+    "    if string is None:\n",
+    "        return None\n",
+    "    string_list = list(string)\n",
+    "    string_length = len(string_list)\n",
+    "    for i in xrange(string_length/2):\n",
+    "        string_list[i], string_list[string_length - 1 - i] = \\\n",
+    "            string_list[string_length - 1 - i], string_list[i]\n",
+    "    return ''.join(string_list)\n",
+    "\n",
+    "run_tests(reverse_string)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## C Algorithm\n",
+    "\n",
+    "This is a classic problem in C/C++\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",
@@ -70,7 +140,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Code"
+    "## C Code"
    ]
   },
   {
@@ -124,24 +194,31 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Pythonic-Code: Not In-Place\n",
+    "## Pythonic-Code\n",
     "\n",
-    "The following code is Pythonic, but requires using additional data structures as Python strings are immutable.  You could use a bytearray or a list instead of a string to simulate manipulating an array of characters."
+    "This question has an artificial constraint that prevented the use of the slice operator and the reversed method.  For completeness, the solutions for these are provided below."
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 7,
    "metadata": {
     "collapsed": false
    },
    "outputs": [],
    "source": [
     "def reverse_string_alt(string):\n",
+    "    if string is None:\n",
+    "        return None\n",
     "    return string[::-1]\n",
     "\n",
     "def reverse_string_alt2(string):\n",
-    "    return ''.join(reversed(string))"
+    "    if string is None:\n",
+    "        return None    \n",
+    "    return ''.join(reversed(string))\n",
+    "\n",
+    "run_tests(reverse_string_alt)\n",
+    "run_tests(reverse_string_alt2)"
    ]
   }
  ],