{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://bit.ly/code-notes)." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.\n", "\n", "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm](#Algorithm)\n", "* [Code](#Code)\n", "* [Unit Test](#Unit-Test)\n", "* [Pythonic-Code](#Pythonic-Code)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints\n", "\n", "*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\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\n", "\n", "### Is Empty\n", "\n", "* Is empty on empty stack\n", "* Is empty 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)\n", "\n", "### Is Empty\n", "* If peek has a value, return False\n", "* Else return True\n", "\n", "Complexity:\n", "* Time: O(1)\n", "* Space: O(1)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Code" ] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Overwriting stack.py\n" ] } ], "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\n", "\n", " def is_empty(self):\n", " return self.peek() is None" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "%run stack.py" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Unit Test\n", "\n", "*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand. You generally will not be asked to write a unit test like what is shown below.*" ] }, { "cell_type": "code", "execution_count": 3, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Test: Empty stack\n", "Test: One element\n", "Test: More than one element\n", "Success: test_end_to_end\n" ] } ], "source": [ "from nose.tools import assert_equal\n", "\n", "class Test(object):\n", " # TODO: It would be better if we had unit tests for each\n", " # method in addition to the following end-to-end test\n", " def test_end_to_end(self):\n", " print('Test: Empty stack')\n", " stack = Stack()\n", " assert_equal(stack.peek(), None)\n", " assert_equal(stack.pop(), None)\n", "\n", " print('Test: One element')\n", " top = Node(5)\n", " stack = Stack(top)\n", " assert_equal(stack.pop(), 5)\n", " assert_equal(stack.peek(), None)\n", "\n", " print('Test: More than one element')\n", " stack = Stack()\n", " stack.push(1)\n", " stack.push(2)\n", " stack.push(3)\n", " assert_equal(stack.pop(), 3)\n", " assert_equal(stack.peek(), 2)\n", " assert_equal(stack.pop(), 2)\n", " assert_equal(stack.peek(), 1)\n", " assert_equal(stack.is_empty(), False)\n", " assert_equal(stack.pop(), 1)\n", " assert_equal(stack.peek(), None)\n", " assert_equal(stack.is_empty(), True)\n", " \n", " print('Success: test_end_to_end')\n", "\n", "if __name__ == '__main__':\n", " test = Test()\n", " test.test_end_to_end()" ] }, { "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": "2.7.10" } }, "nbformat": 4, "nbformat_minor": 0 }