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Added notebook solving the following: Implement a queue using two stacks.
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Donne Martin
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@ -38,6 +38,7 @@ Continually updated IPython Notebooks containing coding problems and solutions (
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* [Implement a stack that keeps track of its minimum element](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/stack-min.ipynb)
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* [Implement a set of stacks class that wraps a list of stacks, each bound by a capacity](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/set-of-stacks.ipynb)
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* [Implement the Towers of Hanoi with 3 towers and N disks](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/hanoi.ipynb)
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* [Implement a queue using two stacks](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/queue-from-stacks.ipynb)
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## Hacker Rank
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208
stacks-queues/queue-from-stacks.ipynb
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208
stacks-queues/queue-from-stacks.ipynb
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@ -0,0 +1,208 @@
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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 using two stacks.\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)"
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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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"* Do you expect the methods to be enqueue and dequeue?\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": 3,
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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": 8,
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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 is_stack_empty(self, stack):\n",
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" return stack.peek() is None\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.is_stack_empty(self.left_stack) and not self.is_stack_empty(self.right_stack):\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.is_stack_empty(self.right_stack) and not self.is_stack_empty(self.left_stack):\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": "code",
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"execution_count": 14,
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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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"Dequeue on empty stack\n",
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"None\n",
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"Enqueue on empty stack\n",
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"Enqueue on non-empty stack\n",
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"Multiple enqueue in a row\n",
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"Dequeue on non-empty stack\n",
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"Dequeue after an enqueue\n",
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"0\n",
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"Multiple dequeue in a row\n",
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"1\n",
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"2\n",
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"Enqueue after a dequeue\n",
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"5\n",
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"None\n"
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]
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}
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],
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"source": [
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"print('Dequeue on empty stack')\n",
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"queue = QueueFromStacks()\n",
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"print(queue.dequeue())\n",
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"print('Enqueue on empty stack')\n",
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"print('Enqueue on non-empty stack')\n",
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"print('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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"print('Dequeue on non-empty stack')\n",
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"print('Dequeue after an enqueue')\n",
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"print(queue.dequeue())\n",
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"print('Multiple dequeue in a row')\n",
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"print(queue.dequeue())\n",
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"print(queue.dequeue())\n",
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"print('Enqueue after a dequeue')\n",
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"queue.enqueue(5)\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": "code",
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"execution_count": null,
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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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}
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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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