Added notebook solving the following: Implement n stacks using a single array.

README.md

@@ -32,6 +32,7 @@ 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)
 
 ## License
 

stacks-queues/n-stacks.ipynb

@@ -0,0 +1,184 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Implement n stacks using a single array.\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",
+    "* Are the stacks and array a fixed size?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* Test the following on the three stacks:\n",
+    "    * Push to full stack\n",
+    "    * Push to non-full stack\n",
+    "    * Pop on empty stack\n",
+    "    * Pop on non-empty stack"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "* Initialize array\n",
+    "* Initialize stack pointers to -1\n",
+    "\n",
+    "### Push\n",
+    "\n",
+    "* If stack is full, throw exception\n",
+    "* Else\n",
+    "    * Increment stack pointer\n",
+    "    * Get the absolute array index\n",
+    "    * Insert the value to this index\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "### Pop\n",
+    "\n",
+    "* If stack is empty, throw exception\n",
+    "* Else\n",
+    "    * Store the value contained in the absolute array index\n",
+    "    * Set the value in the absolute array index to None\n",
+    "    * Decrement stack pointer\n",
+    "    * return value\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "\n",
+    "### Absolute Index\n",
+    "\n",
+    "* return stack size * stack index + stack pointer\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": [
+    "class Stacks(object):\n",
+    "    def __init__(self, num_stacks, stack_size):\n",
+    "        self.num_stacks = num_stacks\n",
+    "        self.stack_size = stack_size\n",
+    "        self.stack_pointers = [-1] * num_stacks\n",
+    "        self.stack_array = [None] * num_stacks * stack_size\n",
+    "\n",
+    "    def abs_index(self, stack_index):\n",
+    "        return stack_index * self.stack_size + self.stack_pointers[stack_index]\n",
+    "\n",
+    "    def push(self, stack_index, data):\n",
+    "        if self.stack_pointers[stack_index] == self.stack_size - 1:\n",
+    "            raise Exception('Stack is full')\n",
+    "        else:\n",
+    "            self.stack_pointers[stack_index] += 1\n",
+    "            array_index = self.abs_index(stack_index)\n",
+    "            self.stack_array[array_index] = data\n",
+    "\n",
+    "    def pop(self, stack_index):\n",
+    "        if self.stack_pointers[stack_index] == -1:\n",
+    "            raise Exception('Stack is empty')\n",
+    "        else:\n",
+    "            array_index = self.abs_index(stack_index)            \n",
+    "            data = self.stack_array[array_index]\n",
+    "            self.stack_array[array_index] = None\n",
+    "            self.stack_pointers[stack_index] -= 1\n",
+    "            return data"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "num_stacks = 3\n",
+    "stack_size = 100\n",
+    "print('Pop on empty stack')\n",
+    "stacks = Stacks(num_stacks, stack_size)\n",
+    "try:\n",
+    "    stacks.pop(0)\n",
+    "except Exception as e:\n",
+    "    print(e)\n",
+    "print('Push to non-full stack')\n",
+    "stacks.push(0, 1)\n",
+    "stacks.push(0, 2)\n",
+    "stacks.push(1, 3)\n",
+    "stacks.push(2, 4)\n",
+    "print('Pop on non-empty stack')\n",
+    "print(stacks.pop(0))\n",
+    "print(stacks.pop(0))\n",
+    "print(stacks.pop(1))\n",
+    "print(stacks.pop(2))\n",
+    "print('Push to full stack')\n",
+    "for i in xrange(0, stack_size):\n",
+    "    stacks.push(2, i)\n",
+    "try:\n",
+    "    stacks.push(2, stack_size)\n",
+    "except Exception as e:\n",
+    "    print(e)"
+   ]
+  }
+ ],
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+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
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