Added notebook solving the following: Implement a queue using two stacks.

README.md

@@ -38,6 +38,7 @@ Continually updated IPython Notebooks containing coding problems and solutions (
 * [Implement a stack that keeps track of its minimum element](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/stack-min.ipynb)
 * [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)
 
 ## Hacker Rank
 

stacks-queues/queue-from-stacks.ipynb

@@ -0,0 +1,208 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement a queue using two stacks.\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",
+    "* Do you expect the methods to be enqueue and dequeue?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* Enqueue and dequeue on empty stack\n",
+    "* Enqueue and dequeue on non-empty stack\n",
+    "* Multiple enqueue in a row\n",
+    "* Multiple dequeue in a row\n",
+    "* Enqueue after a dequeue\n",
+    "* Dequeue after an enqueue"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll use two stacks (left and right) to implement the queue.  The left stack will be used for enqueue and the right stack will be used for dequeue.\n",
+    "\n",
+    "To prevent multiple dequeue calls from needlessly shifting elements around between the stacks, we'll shift elements in a lazy manner.\n",
+    "\n",
+    "### Enqueue\n",
+    "\n",
+    "* If the left stack is empty and the right stack is not empty\n",
+    "    * Shift the elements of the right stack to the left stack\n",
+    "* Push the data to the left stack\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n)\n",
+    "\n",
+    "### Dequeue\n",
+    "\n",
+    "* If the right stack is empty and the the left stack is not empty\n",
+    "    * Shift the elements of the left stack to the right stack\n",
+    "* Pop from the right stack and return the data\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n)\n",
+    "\n",
+    "### Shift Stacks\n",
+    "\n",
+    "* While the source stack has elements:\n",
+    "    * Pop from the source stack and push the data to the destination stack\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(1)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%run stack.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class QueueFromStacks(object):\n",
+    "    def __init__(self):\n",
+    "        self.left_stack = Stack()\n",
+    "        self.right_stack = Stack()\n",
+    "\n",
+    "    def is_stack_empty(self, stack):\n",
+    "        return stack.peek() is None\n",
+    "\n",
+    "    def shift_stacks(self, source, destination):\n",
+    "        while source.peek() is not None:\n",
+    "            destination.push(source.pop())\n",
+    "\n",
+    "    def enqueue(self, data):\n",
+    "        if self.is_stack_empty(self.left_stack) and not self.is_stack_empty(self.right_stack):\n",
+    "            self.shift_stacks(self.right_stack, self.left_stack)\n",
+    "        self.left_stack.push(data)\n",
+    "\n",
+    "    def dequeue(self):\n",
+    "        if self.is_stack_empty(self.right_stack) and not self.is_stack_empty(self.left_stack):\n",
+    "            self.shift_stacks(self.left_stack, self.right_stack)\n",
+    "        return self.right_stack.pop()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 14,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Dequeue on empty stack\n",
+      "None\n",
+      "Enqueue on empty stack\n",
+      "Enqueue on non-empty stack\n",
+      "Multiple enqueue in a row\n",
+      "Dequeue on non-empty stack\n",
+      "Dequeue after an enqueue\n",
+      "0\n",
+      "Multiple dequeue in a row\n",
+      "1\n",
+      "2\n",
+      "Enqueue after a dequeue\n",
+      "5\n",
+      "None\n"
+     ]
+    }
+   ],
+   "source": [
+    "print('Dequeue on empty stack')\n",
+    "queue = QueueFromStacks()\n",
+    "print(queue.dequeue())\n",
+    "print('Enqueue on empty stack')\n",
+    "print('Enqueue on non-empty stack')\n",
+    "print('Multiple enqueue in a row')\n",
+    "num_items = 3\n",
+    "for i in range (0, num_items):\n",
+    "    queue.enqueue(i)\n",
+    "print('Dequeue on non-empty stack')\n",
+    "print('Dequeue after an enqueue')\n",
+    "print(queue.dequeue())\n",
+    "print('Multiple dequeue in a row')\n",
+    "print(queue.dequeue())\n",
+    "print(queue.dequeue())\n",
+    "print('Enqueue after a dequeue')\n",
+    "queue.enqueue(5)\n",
+    "print(queue.dequeue())"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "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
+}