Added notebook solving the following: Sort a stack using another stack as a buffer.

README.md

@@ -39,6 +39,7 @@ Continually updated IPython Notebooks containing coding problems and solutions (
 * [Implement a set of stacks class that wraps a list of stacks, each bound by a capacity](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/set-of-stacks.ipynb)
 * [Implement the Towers of Hanoi with 3 towers and N disks](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/hanoi.ipynb)
 * [Implement a queue using two stacks](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/queue-from-stacks.ipynb)
+* [Sort a stack using another stack as a buffer](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/sort-stack.ipynb)
 
 ## Hacker Rank
 

stacks-queues/sort-stack.ipynb

@@ -0,0 +1,145 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Sort a stack.  You can use another stack as a buffer.\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",
+    "* When sorted, should the largest element be at the top or bottom?\n",
+    "    * Top\n",
+    "* Can you have duplicate values, saw two 5s?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* Empty stack\n",
+    "* One element stack\n",
+    "* Two or more element stack (general case)\n",
+    "* Already sorted stack"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "* Our buffer will hold elements in reverse sorted order, smallest at the top\n",
+    "* Store the current top element in a temp variable\n",
+    "* While stack is not empty\n",
+    "    * While buffer is empty or buffer top is > than temp\n",
+    "        * Move buffer top to stack\n",
+    "    * Move temp to top of buffer\n",
+    "* Return buffer\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n^2)\n",
+    "* Space: O(n)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%run stack.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class MyStack(Stack):\n",
+    "    def sort(self):\n",
+    "        buff = MyStack()\n",
+    "        while not self.is_empty():\n",
+    "            temp = self.pop()\n",
+    "            while not buff.is_empty() and buff.peek() > temp:\n",
+    "                self.push(buff.pop())\n",
+    "            buff.push(temp)\n",
+    "        return buff"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "from random import randint\n",
+    "\n",
+    "def test_sort(num_items, stack=None):\n",
+    "    if stack is None:\n",
+    "        stack = MyStack()\n",
+    "        for _ in xrange(0, num_items):\n",
+    "            stack.push(randint(0, num_items))\n",
+    "    sorted_stack = stack.sort()\n",
+    "    for _ in xrange(0, num_items):\n",
+    "        print(sorted_stack.pop())\n",
+    "    return sorted_stack\n",
+    "\n",
+    "print('Empty stack')\n",
+    "test_sort(0)\n",
+    "print('One element stack')\n",
+    "test_sort(1)\n",
+    "print('Two or more element stack (general case)')\n",
+    "sorted_stack = test_sort(5)"
+   ]
+  }
+ ],
+ "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
+}