Added notebook solving the following: Implement a stack with push, pop, and min methods running O(1) time.

README.md

@@ -33,6 +33,7 @@ Continually updated IPython Notebooks containing coding problems and solutions (
 * [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)
 * [Implement a queue with enqueue and dequeue methods using a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/queue.ipynb)
 * [Implement n stacks using a single array](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/stacks-queues/n-stacks.ipynb)
+* [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)
 
 ## License
 

stacks-queues/stack-min.ipynb

@@ -0,0 +1,175 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement a stack with push, pop, and min methods running O(1) time.\n",
+    "\n",
+    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Pythonic-Code](#Pythonic-Code)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Clarifying Questions\n",
+    "\n",
+    "* Is this a stack of ints?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* Push/pop on empty stack\n",
+    "* Push/pop on non-empty stack"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll use a second stack to keep track of the minimum values.\n",
+    "\n",
+    "### Min\n",
+    "\n",
+    "* If the second stack is empty, return the max int value\n",
+    "* Else, return the top of the stack, without popping it\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "### Push\n",
+    "\n",
+    "* Push the data\n",
+    "* If the data is less than min\n",
+    "    * Push data to second stack\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(n)\n",
+    "\n",
+    "### Pop\n",
+    "\n",
+    "* Pop the data\n",
+    "* If the data is equal to min\n",
+    "    * Pop the top of the second stack\n",
+    "* Return the data\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%run stack.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "import sys\n",
+    "\n",
+    "class MyStack(Stack):\n",
+    "    def __init__(self, top=None):\n",
+    "        self.min_vals = Stack()\n",
+    "        super(MyStack, self).__init__(top)\n",
+    "\n",
+    "    def min(self):\n",
+    "        if self.min_vals.top is None:\n",
+    "            return sys.maxint\n",
+    "        else:\n",
+    "            return self.min_vals.peek()\n",
+    "\n",
+    "    def push(self, data):\n",
+    "        super(MyStack, self).push(data)\n",
+    "        if data < self.min():\n",
+    "            self.min_vals.push(data)\n",
+    "\n",
+    "    def pop(self):\n",
+    "        data = super(MyStack, self).pop()\n",
+    "        if data == self.min():\n",
+    "            self.min_vals.pop()\n",
+    "        return data"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "print('Push on empty stack, non-empty stack')\n",
+    "stack = MyStack()\n",
+    "stack.push(5)\n",
+    "print('Push:', stack.peek(), 'Min after push:', stack.min())\n",
+    "stack.push(1)\n",
+    "print('Push:', stack.peek(), 'Min after push:', stack.min())\n",
+    "stack.push(3)\n",
+    "print('Push:', stack.peek(), 'Min after push:', stack.min())\n",
+    "stack.push(0)\n",
+    "print('Push:', stack.peek(), 'Min after push:', stack.min())\n",
+    "print('Pop on non-empty stack')\n",
+    "print('Pop:', stack.pop(), 'Min after pop:', stack.min())\n",
+    "print('Pop:', stack.pop(), 'Min after pop:', stack.min())\n",
+    "print('Pop:', stack.pop(), 'Min after pop:', stack.min())\n",
+    "print('Pop:', stack.pop(), 'Min after pop:', stack.min())\n",
+    "print('Stack contents:', stack.peek())\n",
+    "print('Pop empty stack:', stack.pop())"
+   ]
+  }
+ ],
+ "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": "2.7.9"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}