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https://github.com/donnemartin/interactive-coding-challenges
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Restructured packages.
This commit is contained in:
Donne Martin
226
stacks_queues/queue_from_stacks/queue_from_stacks.ipynb
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226
stacks_queues/queue_from_stacks/queue_from_stacks.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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"<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>"
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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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"## Problem: Implement a queue using two stacks.\n",
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"\n",
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"* [Constraints and Assumptions](#Constraints-and-Assumptions)\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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"* [Unit Test](#Unit-Test)"
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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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"## Constraints and Assumptions\n",
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"\n",
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"*Problem statements are often intentionally ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
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"\n",
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"* Do you expect the methods to be enqueue and dequeue?\n",
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" * Yes\n",
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"* Can we assume we already have a stack class that can be used for this problem?\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 and dequeue on empty stack\n",
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"* Enqueue and dequeue on non-empty stack\n",
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"* Multiple enqueue in a row\n",
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"* Multiple dequeue in a row\n",
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"* Enqueue after a dequeue\n",
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"* Dequeue after an enqueue"
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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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"We'll use two stacks (left and right) to implement the queue. The left stack will be used for enqueue and the right stack will be used for dequeue.\n",
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"\n",
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"To prevent multiple dequeue calls from needlessly shifting elements around between the stacks, we'll shift elements in a lazy manner.\n",
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"\n",
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"### Enqueue\n",
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"\n",
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"* If the left stack is empty and the right stack is not empty\n",
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" * Shift the elements of the right stack to the left stack\n",
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"* Push the data to the left stack\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\n",
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"* Space: O(n)\n",
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"\n",
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"### Dequeue\n",
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"\n",
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"* If the right stack is empty and the the left stack is not empty\n",
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" * Shift the elements of the left stack to the right stack\n",
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"* Pop from the right stack and return the data\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\n",
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"* Space: O(n)\n",
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"\n",
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"### Shift Stacks\n",
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"\n",
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"* While the source stack has elements:\n",
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" * Pop from the source stack and push the data to the destination stack\n",
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"\n",
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"Complexity:\n",
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"* Time: O(n)\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": 1,
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"metadata": {
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"collapsed": true
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},
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"outputs": [],
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"source": [
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"%run stack.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": 2,
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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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"class QueueFromStacks(object):\n",
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" def __init__(self):\n",
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" self.left_stack = Stack()\n",
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" self.right_stack = Stack()\n",
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"\n",
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" def shift_stacks(self, source, destination):\n",
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" while source.peek() is not None:\n",
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" destination.push(source.pop())\n",
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"\n",
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" def enqueue(self, data):\n",
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" if self.left_stack.is_empty() and not self.right_stack.is_empty():\n",
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" self.shift_stacks(self.right_stack, self.left_stack)\n",
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" self.left_stack.push(data)\n",
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"\n",
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" def dequeue(self):\n",
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" if self.right_stack.is_empty() and not self.left_stack.is_empty():\n",
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" self.shift_stacks(self.left_stack, self.right_stack)\n",
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" return self.right_stack.pop()"
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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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"## Unit Test\n",
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"\n",
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"*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.*"
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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": 3,
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"metadata": {
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"collapsed": false
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},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"Test: Dequeue on empty stack\n",
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"Test: Enqueue on empty stack\n",
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"Test: Enqueue on non-empty stack\n",
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"Test: Multiple enqueue in a row\n",
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"Test: Dequeue on non-empty stack\n",
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"Test: Dequeue after an enqueue\n",
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"Test: Multiple dequeue in a row\n",
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"Test: Enqueue after a dequeue\n",
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"Success: test_queue_from_stacks\n"
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]
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}
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],
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"source": [
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"from nose.tools import assert_equal\n",
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"\n",
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"class Test(object):\n",
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" def test_queue_from_stacks(self):\n",
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" print('Test: Dequeue on empty stack')\n",
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" queue = QueueFromStacks()\n",
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" assert_equal(queue.dequeue(), None)\n",
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"\n",
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" print('Test: Enqueue on empty stack')\n",
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" print('Test: Enqueue on non-empty stack')\n",
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" print('Test: Multiple enqueue in a row')\n",
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" num_items = 3\n",
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" for i in range (0, num_items):\n",
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" queue.enqueue(i)\n",
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"\n",
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" print('Test: Dequeue on non-empty stack')\n",
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" print('Test: Dequeue after an enqueue')\n",
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" assert_equal(queue.dequeue(), 0)\n",
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"\n",
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" print('Test: Multiple dequeue in a row')\n",
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" assert_equal(queue.dequeue(), 1)\n",
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" assert_equal(queue.dequeue(), 2)\n",
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"\n",
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" print('Test: Enqueue after a dequeue')\n",
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" queue.enqueue(5)\n",
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" assert_equal(queue.dequeue(), 5)\n",
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"\n",
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" print('Success: test_queue_from_stacks')\n",
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"\n",
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"if __name__ == '__main__':\n",
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" test = Test()\n",
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" test.test_queue_from_stacks()"
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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": "2.7.10"
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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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