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Added notebook solving the following: Implement a stack with push, pop, and peek methods using a linked list.
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Donne Martin
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@ -28,6 +28,10 @@ Continually updated IPython Notebooks containing coding problems and solutions (
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* [Partition a linked list around a given value](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/partition.ipynb)
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* [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)
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## Stacks and Queues
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* [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)
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## License
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Copyright 2015 Donne Martin
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227
stacks-queues/stack.ipynb
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227
stacks-queues/stack.ipynb
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@ -0,0 +1,227 @@
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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 stack with push, pop, and peek methods using a linked list.\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)\n",
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"* [Pythonic-Code](#Pythonic-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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"* None"
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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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"### Push\n",
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"\n",
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"* Push to empty stack\n",
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"* Push to non-empty stack\n",
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"\n",
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"### Pop\n",
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"\n",
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"* Pop on empty stack\n",
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"* Pop on single element stack\n",
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"* Pop on multiple element stack\n",
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"\n",
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"### Peek\n",
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"\n",
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"* Peek on empty stack\n",
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"* Peek on one or more element stack"
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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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"### Push\n",
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"\n",
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"* Create new node with value\n",
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"* Set node's next to top\n",
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"* Set top to node\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\n",
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"* Space: O(1)\n",
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"\n",
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"### Pop\n",
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"\n",
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"* If stack is empty, return NULL\n",
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"* Else \n",
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" * Save top's value\n",
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" * Set top to top.next\n",
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" * Return saved value\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\n",
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"* Space: O(1)\n",
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"\n",
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"### Peek\n",
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"\n",
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"* If stack is empty, return NULL\n",
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"* Else return top's value\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\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": null,
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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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"%%writefile stack.py\n",
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"\n",
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"class Node(object):\n",
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" def __init__(self, data):\n",
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" self.data = data\n",
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" self.next = None\n",
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"\n",
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"class Stack(object):\n",
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" def __init__(self, top=None):\n",
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" self.top = top\n",
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"\n",
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" def push(self, data):\n",
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" node = Node(data)\n",
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" node.next = self.top\n",
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" self.top = node\n",
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"\n",
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" def pop(self):\n",
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" if self.top is not None:\n",
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" data = self.top.data\n",
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" self.top = self.top.next\n",
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" return data\n",
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" return None\n",
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"\n",
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" def peek(self):\n",
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" if self.top is not None:\n",
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" return self.top.data\n",
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" return None"
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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": false
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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": null,
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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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"print('Empty stack')\n",
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"stack = Stack()\n",
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"print(stack.peek())\n",
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"print(stack.pop())\n",
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"print('One element')\n",
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"top = Node(5)\n",
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"stack = Stack(top)\n",
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"print(stack.pop())\n",
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"print(stack.peek())\n",
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"print('More than one element')\n",
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"stack = Stack()\n",
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"stack.push(1)\n",
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"stack.push(2)\n",
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"stack.push(3)\n",
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"print(stack.pop())\n",
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"print(stack.peek())\n",
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"print(stack.pop())\n",
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"print(stack.peek())\n",
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"print(stack.pop())\n",
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"print(stack.peek())"
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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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"## Pythonic-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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"Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks\n",
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"<pre>\n",
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"5.1.1. Using Lists as Stacks\n",
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"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",
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"\n",
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">>> stack = [3, 4, 5]\n",
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">>> stack.append(6)\n",
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">>> stack.append(7)\n",
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">>> stack\n",
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"[3, 4, 5, 6, 7]\n",
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">>> stack.pop()\n",
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"7\n",
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">>> stack\n",
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"[3, 4, 5, 6]\n",
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">>> stack.pop()\n",
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"6\n",
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">>> stack.pop()\n",
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"5\n",
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">>> stack\n",
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"[3, 4]\n",
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"</pre>"
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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": "2.7.9"
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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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26
stacks-queues/stack.py
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26
stacks-queues/stack.py
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@ -0,0 +1,26 @@
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class Node(object):
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def __init__(self, data):
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self.data = data
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self.next = None
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class Stack(object):
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def __init__(self, top=None):
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self.top = top
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def push(self, data):
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node = Node(data)
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node.next = self.top
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self.top = node
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def pop(self):
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if self.top is not None:
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data = self.top.data
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self.top = self.top.next
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return data
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return None
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def peek(self):
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if self.top is not None:
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return self.top.data
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return None
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