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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/stack_min/stack_min.ipynb
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226
stacks_queues/stack_min/stack_min.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 stack with push, pop, and min methods running O(1) time.\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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"* Can we assume this is a stack of ints?\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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"* Push/pop on empty stack\n",
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"* Push/pop on non-empty 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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"We'll use a second stack to keep track of the minimum values.\n",
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"\n",
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"### Min\n",
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"\n",
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"* If the second stack is empty, return an error code (max int value)\n",
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"* Else, return the top of the stack, without popping it\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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"### Push\n",
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"\n",
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"* Push the data\n",
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"* If the data is less than min\n",
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" * Push data to second stack\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(n)\n",
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"\n",
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"### Pop\n",
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"\n",
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"* Pop the data\n",
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"* If the data is equal to min\n",
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" * Pop the top of the second stack\n",
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"* Return the data\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": 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": true
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},
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"outputs": [],
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"source": [
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"import sys\n",
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"\n",
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"class MyStack(Stack):\n",
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" def __init__(self, top=None):\n",
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" self.min_vals = Stack()\n",
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" super(MyStack, self).__init__(top)\n",
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"\n",
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" def min(self):\n",
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" if self.min_vals.top is None:\n",
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" return sys.maxint\n",
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" else:\n",
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" return self.min_vals.peek()\n",
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"\n",
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" def push(self, data):\n",
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" super(MyStack, self).push(data)\n",
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" if data < self.min():\n",
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" self.min_vals.push(data)\n",
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"\n",
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" def pop(self):\n",
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" data = super(MyStack, self).pop()\n",
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" if data == self.min():\n",
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" self.min_vals.pop()\n",
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" return data"
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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: Push on empty stack, non-empty stack\n",
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"Test: Pop on non-empty stack\n",
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"Test: Pop empty stack\n",
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"Success: test_stack_min\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_stack_min(self):\n",
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" print('Test: Push on empty stack, non-empty stack')\n",
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" stack = MyStack()\n",
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" stack.push(5)\n",
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" assert_equal(stack.peek(), 5)\n",
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" assert_equal(stack.min(), 5)\n",
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" stack.push(1)\n",
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" assert_equal(stack.peek(), 1)\n",
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" assert_equal(stack.min(), 1)\n",
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" stack.push(3)\n",
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" assert_equal(stack.peek(), 3)\n",
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" assert_equal(stack.min(), 1)\n",
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" stack.push(0)\n",
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" assert_equal(stack.peek(), 0)\n",
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" assert_equal(stack.min(), 0)\n",
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"\n",
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" print('Test: Pop on non-empty stack')\n",
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" assert_equal(stack.pop(), 0)\n",
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" assert_equal(stack.min(), 1)\n",
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" assert_equal(stack.pop(), 3)\n",
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" assert_equal(stack.min(), 1)\n",
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" assert_equal(stack.pop(), 1)\n",
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" assert_equal(stack.min(), 5)\n",
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" assert_equal(stack.pop(), 5)\n",
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" assert_equal(stack.min(), sys.maxint)\n",
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"\n",
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" print('Test: Pop empty stack')\n",
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" assert_equal(stack.pop(), None)\n",
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" \n",
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" print('Success: test_stack_min')\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_stack_min()"
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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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