{ "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 SetOfStacks that wraps a list of stacks, where each stack is bound by a capacity.\n", "\n", "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n", "* [Test Cases](#Test-Cases)\n", "* [Algorithm](#Algorithm)\n", "* [Code](#Code)\n", "* [Unit Test](#Unit-Test)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Constraints and Assumptions\n", "\n", "*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n", "\n", "* Can we assume we already have a stack class that can be used for this problem?\n", " * Yes\n", "* If a stack becomes full, we should automatically create one?\n", " * Yes\n", "* If a stack becomes empty, should we delete it?\n", " * Yes" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Test Cases\n", "\n", "* Push and pop on an empty stack\n", "* Push and pop on a non-empty stack\n", "* Push on a capacity stack to create a new one\n", "* Pop on a one element stack to destroy it" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Algorithm\n", "\n", "### Push\n", "\n", "* If there are no stacks or the last stack is full\n", " * Create a new stack\n", "* Push to the new stack\n", "\n", "Complexity:\n", "* Time: O(1)\n", "* Space: O(1)\n", "\n", "### Pop\n", "\n", "* If there are no stacks, return None\n", "* Else if the last stack has one element\n", " * Pop the last element's data\n", " * Delete the now empty stack\n", " * Update the last stack pointer\n", "* Else Pop the last element's data\n", "* Return the last element's data\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": true }, "outputs": [], "source": [ "%run stack.py" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [], "source": [ "class StackWithCapacity(Stack):\n", " def __init__(self, top=None, capacity=10):\n", " self.capacity = capacity\n", " self.num_items = 0\n", " super(StackWithCapacity, self).__init__(top)\n", "\n", " def push(self, data):\n", " if self.num_items < self.capacity:\n", " super(StackWithCapacity, self).push(data)\n", " self.num_items += 1\n", " else:\n", " raise Exception('Stack full')\n", "\n", " def pop(self):\n", " data = super(StackWithCapacity, self).pop()\n", " self.num_items -= 1\n", " return data\n", "\n", " def is_full(self):\n", " return self.num_items == self.capacity\n", "\n", "class SetOfStacks(object):\n", " def __init__(self, capacity):\n", " self.capacity = capacity\n", " self.stacks = []\n", " self.last_stack = None\n", "\n", " def push(self, data):\n", " if self.last_stack is None or self.last_stack.is_full():\n", " self.last_stack = StackWithCapacity(None, self.capacity)\n", " self.stacks.append(self.last_stack)\n", " self.last_stack.push(data)\n", "\n", " def pop(self):\n", " if self.last_stack is None:\n", " return\n", " elif self.last_stack.top.next is None:\n", " data = self.last_stack.pop()\n", " self.stacks.pop()\n", " num_stacks = len(self.stacks)\n", " if num_stacks == 0:\n", " self.last_stack = None\n", " else:\n", " self.last_stack = self.stacks[num_stacks-1]\n", " else:\n", " data = self.last_stack.pop()\n", " return data" ] }, { "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: Push on an empty stack\n", "Test: Push on a non-empty stack\n", "Test: Push on a capacity stack to create a new one\n", "Test: Pop on a one element stack to destroy it\n", "Test: Pop general case\n", "Test: Pop on no elements\n", "Success: test_set_of_stacks\n" ] } ], "source": [ "from nose.tools import assert_equal\n", "\n", "class Test(object):\n", " def test_set_of_stacks(self):\n", " print('Test: Push on an empty stack')\n", " capacity = 2\n", " stacks = SetOfStacks(capacity)\n", " stacks.push(3)\n", "\n", " print('Test: Push on a non-empty stack')\n", " stacks.push(5)\n", "\n", " print('Test: Push on a capacity stack to create a new one')\n", " stacks.push('a')\n", "\n", " print('Test: Pop on a one element stack to destroy it')\n", " assert_equal(stacks.pop(), 'a')\n", "\n", " print('Test: Pop general case')\n", " assert_equal(stacks.pop(), 5)\n", " assert_equal(stacks.pop(), 3)\n", "\n", " print('Test: Pop on no elements')\n", " assert_equal(stacks.pop(), None)\n", " \n", " print('Success: test_set_of_stacks')\n", "\n", "if __name__ == '__main__':\n", " test = Test()\n", " test.test_set_of_stacks()" ] } ], "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 }