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Added notebook solving the following: Implement a queue with enqueue and dequeue methods using a linked list.
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Donne Martin
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@ -30,7 +30,8 @@ Continually updated IPython Notebooks containing coding problems and solutions (
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## Stacks and Queues
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* [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)
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* [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)
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* [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)
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## License
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219
stacks-queues/queue.ipynb
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219
stacks-queues/queue.ipynb
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Problem: Implement a queue with enqueue and dequeue methods using a linked list.\n",
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"\n",
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"* [Clarifying Questions](#Clarifying-Questions)\n",
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"* [Test Cases](#Test-Cases)\n",
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"* [Algorithm](#Algorithm)\n",
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"* [Code](#Code)\n",
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"* [Pythonic-Code](#Pythonic-Code)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Clarifying Questions\n",
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"\n",
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"* If there is one item in the list, do you expect the first and last pointers to both point to it?\n",
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" * Yes\n",
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"* If there are no items on the list, do you expect the first and last pointers to be NULL?\n",
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" * Yes"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Test Cases\n",
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"\n",
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"### Enqueue\n",
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"\n",
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"* Enqueue to an empty queue\n",
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"* Enqueue to a non-empty queue\n",
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"\n",
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"### Dequeue\n",
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"\n",
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"* Dequeue an empty queue\n",
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"* Dequeue a queue with one element\n",
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"* Dequeue a queue with more than one element"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Algorithm\n",
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"\n",
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"### Enqueue\n",
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"\n",
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"* If the list is empty, set first and last to node\n",
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"* Else, set last to node\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\n",
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"* Space: O(1)\n",
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"\n",
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"### Dequeue\n",
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"\n",
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"* If the list is empty, return NULL\n",
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"* If the list has one node\n",
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" * Save the first node's value\n",
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" * Set first and last to NULL\n",
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" * Return the saved value\n",
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"* Else\n",
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" * Save the first node's value\n",
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" * Set first to its next node\n",
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" * Return the saved value\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\n",
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"* Space: O(1)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Code"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"%%writefile queue.py\n",
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"\n",
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"class Node(object):\n",
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" def __init__(self, data):\n",
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" self.data = data\n",
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" self.next = None\n",
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"\n",
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"class Queue(object):\n",
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" def __init__(self):\n",
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" self.first = None\n",
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" self.last = None\n",
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"\n",
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" def enqueue(self, data):\n",
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" node = Node(data)\n",
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" if self.first is None and self.last is None:\n",
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" self.first = node\n",
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" self.last = node\n",
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" else:\n",
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" self.last.next = node\n",
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" self.last = node\n",
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"\n",
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" def dequeue(self):\n",
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" # Empty list\n",
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" if self.first is None and self.last is None:\n",
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" return None\n",
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" # Remove only element from a one element list\n",
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" elif self.first == self.last:\n",
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" data = self.first.data\n",
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" self.first = None\n",
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" self.last = None\n",
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" return data\n",
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" else:\n",
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" data = self.first.data\n",
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" self.first = self.first.next\n",
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" return data"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"%run queue.py"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"print('Dequeue an empty queue')\n",
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"queue = Queue()\n",
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"print(queue.dequeue())\n",
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"print('Enqueue to an empty queue')\n",
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"queue.enqueue(1)\n",
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"print('Dequeue a queue with one element')\n",
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"print(queue.dequeue())\n",
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"print('Enqueue to a non-empty queue')\n",
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"queue.enqueue(2)\n",
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"queue.enqueue(3)\n",
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"queue.enqueue(4)\n",
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"print('Dequeue a queue with more than one element')\n",
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"print(queue.dequeue())\n",
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"print(queue.dequeue())\n",
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"print(queue.dequeue())"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Pythonic-Code"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-queues\n",
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"<pre>\n",
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"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",
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"\n",
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"To implement a queue, use collections.deque which was designed to have fast appends and pops from both ends. For example:\n",
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"\n",
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">>>\n",
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">>> from collections import deque\n",
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">>> queue = deque([\"Eric\", \"John\", \"Michael\"])\n",
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">>> queue.append(\"Terry\") # Terry arrives\n",
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">>> queue.append(\"Graham\") # Graham arrives\n",
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">>> queue.popleft() # The first to arrive now leaves\n",
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"'Eric'\n",
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">>> queue.popleft() # The second to arrive now leaves\n",
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"'John'\n",
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">>> queue # Remaining queue in order of arrival\n",
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"deque(['Michael', 'Terry', 'Graham'])\n",
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"</pre>"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 2",
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"language": "python",
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"name": "python2"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 2
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython2",
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"version": "2.7.9"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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34
stacks-queues/queue.py
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34
stacks-queues/queue.py
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@ -0,0 +1,34 @@
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class Node(object):
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def __init__(self, data):
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self.data = data
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self.next = None
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class Queue(object):
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def __init__(self):
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self.first = None
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self.last = None
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def enqueue(self, data):
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node = Node(data)
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if self.first is None and self.last is None:
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self.first = node
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self.last = node
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else:
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self.last.next = node
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self.last = node
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def dequeue(self):
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# Empty list
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if self.first is None and self.last is None:
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return None
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# Remove only element from a one element list
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elif self.first == self.last:
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data = self.first.data
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self.first = None
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self.last = None
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return data
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else:
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data = self.first.data
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self.first = self.first.next
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return data
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