interactive-coding-challenges/stacks_queues/queue_list/queue_list.ipynb

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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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    "## Problem: Implement a queue with enqueue and dequeue methods using a linked list.\n",
    "\n",
    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)\n",
    "* [Pythonic-Code](#Pythonic-Code)"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Constraints\n",
    "\n",
    "*Problem statements are often intentionally ambiguous.  Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
    "\n",
    "* If there is one item in the list, do you expect the first and last pointers to both point to it?\n",
    "    * Yes\n",
    "* If there are no items on the list, do you expect the first and last pointers to be NULL?\n",
    "    * Yes"
   ]
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   "source": [
    "## Test Cases\n",
    "\n",
    "### Enqueue\n",
    "\n",
    "* Enqueue to an empty queue\n",
    "* Enqueue to a non-empty queue\n",
    "\n",
    "### Dequeue\n",
    "\n",
    "* Dequeue an empty queue\n",
    "* Dequeue a queue with one element\n",
    "* Dequeue a queue with more than one element"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "### Enqueue\n",
    "\n",
    "* If the list is empty, set first and last to node\n",
    "* Else, set last to node\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)\n",
    "\n",
    "### Dequeue\n",
    "\n",
    "* If the list is empty, return NULL\n",
    "* If the list has one node\n",
    "    * Save the first node's value\n",
    "    * Set first and last to NULL\n",
    "    * Return the saved value\n",
    "* Else\n",
    "    * Save the first node's value\n",
    "    * Set first to its next node\n",
    "    * Return the saved value\n",
    "\n",
    "Complexity:\n",
    "* Time: O(1)\n",
    "* Space: O(1)"
   ]
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   "source": [
    "## Code"
   ]
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  {
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   "execution_count": 1,
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    {
     "name": "stdout",
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     "text": [
      "Overwriting queue_list.py\n"
     ]
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   "source": [
    "%%writefile queue_list.py\n",
    "# Need to avoid naming this queue.py as it \n",
    "# will conflict with IPython Notebook\n",
    "\n",
    "class Node(object):\n",
    "    def __init__(self, data):\n",
    "        self.data = data\n",
    "        self.next = None\n",
    "\n",
    "class Queue(object):\n",
    "    def __init__(self):\n",
    "        self.first = None\n",
    "        self.last = None\n",
    "\n",
    "    def enqueue(self, data):\n",
    "        node = Node(data)\n",
    "        if self.first is None and self.last is None:\n",
    "            self.first = node\n",
    "            self.last = node\n",
    "        else:\n",
    "            self.last.next = node\n",
    "            self.last = node\n",
    "\n",
    "    def dequeue(self):\n",
    "        # Empty list\n",
    "        if self.first is None and self.last is None:\n",
    "            return None\n",
    "        \n",
    "        # Remove only element from a one element list\n",
    "        elif self.first == self.last:\n",
    "            data = self.first.data\n",
    "            self.first = None\n",
    "            self.last = None\n",
    "            return data\n",
    "        else:\n",
    "            data = self.first.data\n",
    "            self.first = self.first.next\n",
    "            return data"
   ]
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   "cell_type": "code",
   "execution_count": 2,
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   "source": [
    "%run queue_list.py"
   ]
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   "metadata": {},
   "source": [
    "## Unit Test\n",
    "\n",
    "*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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   "cell_type": "code",
   "execution_count": 3,
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     "text": [
      "Test: Dequeue an empty queue\n",
      "Test: Enqueue to an empty queue\n",
      "Test: Dequeue a queue with one element\n",
      "Test: Enqueue to a non-empty queue\n",
      "Test: Dequeue a queue with more than one element\n",
      "Success: test_end_to_end\n"
     ]
    }
   ],
   "source": [
    "from nose.tools import assert_equal\n",
    "\n",
    "class Test(object):\n",
    "    # TODO: It would be better if we had unit tests for each\n",
    "    # method in addition to the following end-to-end test\n",
    "    def test_end_to_end(self):\n",
    "        print('Test: Dequeue an empty queue')\n",
    "        queue = Queue()\n",
    "        assert_equal(queue.dequeue(), None)\n",
    "\n",
    "        print('Test: Enqueue to an empty queue')\n",
    "        queue.enqueue(1)\n",
    "\n",
    "        print('Test: Dequeue a queue with one element')\n",
    "        assert_equal(queue.dequeue(), 1)\n",
    "\n",
    "        print('Test: Enqueue to a non-empty queue')\n",
    "        queue.enqueue(2)\n",
    "        queue.enqueue(3)\n",
    "        queue.enqueue(4)\n",
    "\n",
    "        print('Test: Dequeue a queue with more than one element')\n",
    "        assert_equal(queue.dequeue(), 2)\n",
    "        assert_equal(queue.dequeue(), 3)\n",
    "        assert_equal(queue.dequeue(), 4)\n",
    "        \n",
    "        print('Success: test_end_to_end')\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    test = Test()\n",
    "    test.test_end_to_end()"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pythonic-Code"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-queues\n",
    "<pre>\n",
    "It is possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).\n",
    "\n",
    "To implement a queue, use collections.deque which was designed to have fast appends and pops from both ends. For example:\n",
    "\n",
    ">>>\n",
    ">>> from collections import deque\n",
    ">>> queue = deque([\"Eric\", \"John\", \"Michael\"])\n",
    ">>> queue.append(\"Terry\")           # Terry arrives\n",
    ">>> queue.append(\"Graham\")          # Graham arrives\n",
    ">>> queue.popleft()                 # The first to arrive now leaves\n",
    "'Eric'\n",
    ">>> queue.popleft()                 # The second to arrive now leaves\n",
    "'John'\n",
    ">>> queue                           # Remaining queue in order of arrival\n",
    "deque(['Michael', 'Terry', 'Graham'])\n",
    "</pre>"
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  }
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