From 72ff274ff6672bbd703d837d60ac65017a44f32c Mon Sep 17 00:00:00 2001 From: Donne Martin Date: Fri, 15 May 2015 06:08:46 -0400 Subject: [PATCH] Added notebook solving the following: Implement a stack with push, pop, and peek methods using a linked list. --- README.md | 4 + stacks-queues/stack.ipynb | 227 ++++++++++++++++++++++++++++++++++++++ stacks-queues/stack.py | 26 +++++ 3 files changed, 257 insertions(+) create mode 100644 stacks-queues/stack.ipynb create mode 100644 stacks-queues/stack.py diff --git a/README.md b/README.md index a915477..ea03592 100644 --- a/README.md +++ b/README.md @@ -28,6 +28,10 @@ Continually updated IPython Notebooks containing coding problems and solutions ( * [Partition a linked list around a given value](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/partition.ipynb) * [Add two numbers whose digits are stored in a linked list in reverse order](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/add-reverse.ipynb) +## 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) + ## License Copyright 2015 Donne Martin diff --git a/stacks-queues/stack.ipynb b/stacks-queues/stack.ipynb new file mode 100644 index 0000000..3aa5a35 --- /dev/null +++ b/stacks-queues/stack.ipynb @@ -0,0 +1,227 @@ +{ + "cells": [ + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Problem: Implement a stack with push, pop, and peek 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", + "* None" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Test Cases\n", + "\n", + "### Push\n", + "\n", + "* Push to empty stack\n", + "* Push to non-empty stack\n", + "\n", + "### Pop\n", + "\n", + "* Pop on empty stack\n", + "* Pop on single element stack\n", + "* Pop on multiple element stack\n", + "\n", + "### Peek\n", + "\n", + "* Peek on empty stack\n", + "* Peek on one or more element stack" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Algorithm\n", + "\n", + "### Push\n", + "\n", + "* Create new node with value\n", + "* Set node's next to top\n", + "* Set top to node\n", + "\n", + "Complexity:\n", + "* Time: O(1)\n", + "* Space: O(1)\n", + "\n", + "### Pop\n", + "\n", + "* If stack is empty, return NULL\n", + "* Else \n", + " * Save top's value\n", + " * Set top to top.next\n", + " * Return saved value\n", + "\n", + "Complexity:\n", + "* Time: O(1)\n", + "* Space: O(1)\n", + "\n", + "### Peek\n", + "\n", + "* If stack is empty, return NULL\n", + "* Else return top's 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 stack.py\n", + "\n", + "class Node(object):\n", + " def __init__(self, data):\n", + " self.data = data\n", + " self.next = None\n", + "\n", + "class Stack(object):\n", + " def __init__(self, top=None):\n", + " self.top = top\n", + "\n", + " def push(self, data):\n", + " node = Node(data)\n", + " node.next = self.top\n", + " self.top = node\n", + "\n", + " def pop(self):\n", + " if self.top is not None:\n", + " data = self.top.data\n", + " self.top = self.top.next\n", + " return data\n", + " return None\n", + "\n", + " def peek(self):\n", + " if self.top is not None:\n", + " return self.top.data\n", + " return None" + ] + }, + { + "cell_type": "code", + "execution_count": null, + "metadata": { + "collapsed": false + }, + "outputs": [], + "source": [ + "%run stack.py" + ] + }, + { + "cell_type": "code", + "execution_count": null, + "metadata": { + "collapsed": false + }, + "outputs": [], + "source": [ + "print('Empty stack')\n", + "stack = Stack()\n", + "print(stack.peek())\n", + "print(stack.pop())\n", + "print('One element')\n", + "top = Node(5)\n", + "stack = Stack(top)\n", + "print(stack.pop())\n", + "print(stack.peek())\n", + "print('More than one element')\n", + "stack = Stack()\n", + "stack.push(1)\n", + "stack.push(2)\n", + "stack.push(3)\n", + "print(stack.pop())\n", + "print(stack.peek())\n", + "print(stack.pop())\n", + "print(stack.peek())\n", + "print(stack.pop())\n", + "print(stack.peek())" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "## Pythonic-Code" + ] + }, + { + "cell_type": "markdown", + "metadata": {}, + "source": [ + "Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks\n", + "
\n",
+    "5.1.1. Using Lists as Stacks\n",
+    "The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index. For example:\n",
+    "\n",
+    ">>> stack = [3, 4, 5]\n",
+    ">>> stack.append(6)\n",
+    ">>> stack.append(7)\n",
+    ">>> stack\n",
+    "[3, 4, 5, 6, 7]\n",
+    ">>> stack.pop()\n",
+    "7\n",
+    ">>> stack\n",
+    "[3, 4, 5, 6]\n",
+    ">>> stack.pop()\n",
+    "6\n",
+    ">>> stack.pop()\n",
+    "5\n",
+    ">>> stack\n",
+    "[3, 4]\n",
+    "
" + ] + } + ], + "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 +} diff --git a/stacks-queues/stack.py b/stacks-queues/stack.py new file mode 100644 index 0000000..e8780be --- /dev/null +++ b/stacks-queues/stack.py @@ -0,0 +1,26 @@ + +class Node(object): + def __init__(self, data): + self.data = data + self.next = None + +class Stack(object): + def __init__(self, top=None): + self.top = top + + def push(self, data): + node = Node(data) + node.next = self.top + self.top = node + + def pop(self): + if self.top is not None: + data = self.top.data + self.top = self.top.next + return data + return None + + def peek(self): + if self.top is not None: + return self.top.data + return None \ No newline at end of file