Added notebook solving the following: Implement a queue with enqueue and dequeue methods using a linked list.

README.md

@@ -30,7 +30,8 @@ Continually updated IPython Notebooks containing coding problems and solutions (
 
 ## Stacks and Queues
 
-* [Implement a stack with push, pop, and peek methods using a linked list.](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/stack.ipynb)
+* [Implement a stack with push, pop, and peek methods using a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/stack.ipynb)
+* [Implement a queue with enqueue and dequeue methods using a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/queue.ipynb)
 
 ## License
 

stacks-queues/queue.ipynb

@@ -0,0 +1,219 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement a queue with enqueue and dequeue methods using a linked list.\n",
+    "\n",
+    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Pythonic-Code](#Pythonic-Code)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "\n",
+    "* If there is one item in the list, do you expect the first and last pointers to both point to it?\n",
+    "    * Yes\n",
+    "* If there are no items on the list, do you expect the first and last pointers to be NULL?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "### Enqueue\n",
+    "\n",
+    "* Enqueue to an empty queue\n",
+    "* Enqueue to a non-empty queue\n",
+    "\n",
+    "### Dequeue\n",
+    "\n",
+    "* Dequeue an empty queue\n",
+    "* Dequeue a queue with one element\n",
+    "* Dequeue a queue with more than one element"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "### Enqueue\n",
+    "\n",
+    "* If the list is empty, set first and last to node\n",
+    "* Else, set last to node\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "### Dequeue\n",
+    "\n",
+    "* If the list is empty, return NULL\n",
+    "* If the list has one node\n",
+    "    * Save the first node's value\n",
+    "    * Set first and last to NULL\n",
+    "    * Return the saved value\n",
+    "* Else\n",
+    "    * Save the first node's value\n",
+    "    * Set first to its next node\n",
+    "    * Return the saved value\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "%%writefile queue.py\n",
+    "\n",
+    "class Node(object):\n",
+    "    def __init__(self, data):\n",
+    "        self.data = data\n",
+    "        self.next = None\n",
+    "\n",
+    "class Queue(object):\n",
+    "    def __init__(self):\n",
+    "        self.first = None\n",
+    "        self.last = None\n",
+    "\n",
+    "    def enqueue(self, data):\n",
+    "        node = Node(data)\n",
+    "        if self.first is None and self.last is None:\n",
+    "            self.first = node\n",
+    "            self.last = node\n",
+    "        else:\n",
+    "            self.last.next = node\n",
+    "            self.last = node\n",
+    "\n",
+    "    def dequeue(self):\n",
+    "        # Empty list\n",
+    "        if self.first is None and self.last is None:\n",
+    "            return None\n",
+    "        # Remove only element from a one element list\n",
+    "        elif self.first == self.last:\n",
+    "            data = self.first.data\n",
+    "            self.first = None\n",
+    "            self.last = None\n",
+    "            return data\n",
+    "        else:\n",
+    "            data = self.first.data\n",
+    "            self.first = self.first.next\n",
+    "            return data"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "%run queue.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "print('Dequeue an empty queue')\n",
+    "queue = Queue()\n",
+    "print(queue.dequeue())\n",
+    "print('Enqueue to an empty queue')\n",
+    "queue.enqueue(1)\n",
+    "print('Dequeue a queue with one element')\n",
+    "print(queue.dequeue())\n",
+    "print('Enqueue to a non-empty queue')\n",
+    "queue.enqueue(2)\n",
+    "queue.enqueue(3)\n",
+    "queue.enqueue(4)\n",
+    "print('Dequeue a queue with more than one element')\n",
+    "print(queue.dequeue())\n",
+    "print(queue.dequeue())\n",
+    "print(queue.dequeue())"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Pythonic-Code"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-queues\n",
+    "<pre>\n",
+    "It is possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).\n",
+    "\n",
+    "To implement a queue, use collections.deque which was designed to have fast appends and pops from both ends. For example:\n",
+    "\n",
+    ">>>\n",
+    ">>> from collections import deque\n",
+    ">>> queue = deque([\"Eric\", \"John\", \"Michael\"])\n",
+    ">>> queue.append(\"Terry\")           # Terry arrives\n",
+    ">>> queue.append(\"Graham\")          # Graham arrives\n",
+    ">>> queue.popleft()                 # The first to arrive now leaves\n",
+    "'Eric'\n",
+    ">>> queue.popleft()                 # The second to arrive now leaves\n",
+    "'John'\n",
+    ">>> queue                           # Remaining queue in order of arrival\n",
+    "deque(['Michael', 'Terry', 'Graham'])\n",
+    "</pre>"
+   ]
+  }
+ ],
+ "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
+}

stacks-queues/queue.py

@@ -0,0 +1,34 @@
+
+class Node(object):
+    def __init__(self, data):
+        self.data = data
+        self.next = None
+
+class Queue(object):
+    def __init__(self):
+        self.first = None
+        self.last = None
+
+    def enqueue(self, data):
+        node = Node(data)
+        if self.first is None and self.last is None:
+            self.first = node
+            self.last = node
+        else:
+            self.last.next = node
+            self.last = node
+
+    def dequeue(self):
+        # Empty list
+        if self.first is None and self.last is None:
+            return None
+        # Remove only element from a one element list
+        elif self.first == self.last:
+            data = self.first.data
+            self.first = None
+            self.last = None
+            return data
+        else:
+            data = self.first.data
+            self.first = self.first.next
+            return data