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    "## Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.\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",
    "* None"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "### Push\n",
    "\n",
    "* Push to empty stack\n",
    "* Push to non-empty stack\n",
    "\n",
    "### Pop\n",
    "\n",
    "* Pop on empty stack\n",
    "* Pop on single element stack\n",
    "* Pop on multiple element stack\n",
    "\n",
    "### Peek\n",
    "\n",
    "* Peek on empty stack\n",
    "* Peek on one or more element stack\n",
    "\n",
    "### Is Empty\n",
    "\n",
    "* Is empty on empty stack\n",
    "* Is empty on one or more element stack"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "### Push\n",
    "\n",
    "* Create new node with value\n",
    "* Set node's next to top\n",
    "* Set top to node\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)\n",
    "\n",
    "### Pop\n",
    "\n",
    "* If stack is empty, return NULL\n",
    "* Else \n",
    "    * Save top's value\n",
    "    * Set top to top.next\n",
    "    * Return saved value\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)\n",
    "\n",
    "### Peek\n",
    "\n",
    "* If stack is empty, return NULL\n",
    "* Else return top's value\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)\n",
    "\n",
    "### Is Empty\n",
    "* If peek has a value, return False\n",
    "* Else return True\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)"
   ]
  },
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   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "%%writefile stack.py\n",
    "\n",
    "class Node(object):\n",
    "    def __init__(self, data):\n",
    "        self.data = data\n",
    "        self.next = None\n",
    "\n",
    "class Stack(object):\n",
    "    def __init__(self, top=None):\n",
    "        self.top = top\n",
    "\n",
    "    def push(self, data):\n",
    "        node = Node(data)\n",
    "        node.next = self.top\n",
    "        self.top = node\n",
    "\n",
    "    def pop(self):\n",
    "        if self.top is not None:\n",
    "            data = self.top.data\n",
    "            self.top = self.top.next\n",
    "            return data\n",
    "        return None\n",
    "\n",
    "    def peek(self):\n",
    "        if self.top is not None:\n",
    "            return self.top.data\n",
    "        return None\n",
    "\n",
    "    def is_empty(self):\n",
    "        return self.peek() is None"
   ]
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  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   "outputs": [],
   "source": [
    "%run stack.py"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "print('Empty stack')\n",
    "stack = Stack()\n",
    "print(stack.peek())\n",
    "print(stack.pop())\n",
    "print('One element')\n",
    "top = Node(5)\n",
    "stack = Stack(top)\n",
    "print(stack.pop())\n",
    "print(stack.peek())\n",
    "print('More than one element')\n",
    "stack = Stack()\n",
    "stack.push(1)\n",
    "stack.push(2)\n",
    "stack.push(3)\n",
    "print(stack.pop())\n",
    "print(stack.peek())\n",
    "print(stack.pop())\n",
    "print(stack.peek())\n",
    "print(stack.is_empty())\n",
    "print(stack.pop())\n",
    "print(stack.peek())\n",
    "print(stack.is_empty())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pythonic-Code"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks\n",
    "<pre>\n",
    "5.1.1. Using Lists as Stacks\n",
    "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",
    "\n",
    ">>> stack = [3, 4, 5]\n",
    ">>> stack.append(6)\n",
    ">>> stack.append(7)\n",
    ">>> stack\n",
    "[3, 4, 5, 6, 7]\n",
    ">>> stack.pop()\n",
    "7\n",
    ">>> stack\n",
    "[3, 4, 5, 6]\n",
    ">>> stack.pop()\n",
    "6\n",
    ">>> stack.pop()\n",
    "5\n",
    ">>> stack\n",
    "[3, 4]\n",
    "</pre>"
   ]
  }
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