{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<small><i>This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes).</i></small>"
   ]
  },
  {
   "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_list.py\n",
    "# Need to avoid naming this queue.py as it will conflict with IPython Notebook\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_list.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": "2.7.10"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 0
}