interactive-coding-challenges/linked_lists/add-reverse.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: Add two numbers whose digits are stored in a linked list in reverse order.\n",
    "\n",
    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)\n",
    "* [Unit Test](#Unit-Test)"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Constraints and Assumptions\n",
    "\n",
    "*Problem statements are often intentionally ambiguous.  Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
    "\n",
    "* Do you expect the return to be in reverse order too?\n",
    "    * Yes\n",
    "* What if one of the inputs is NULL?\n",
    "    * Return NULL for an invalid operation\n",
    "* How large are these numbers--can they fit in memory?\n",
    "    * Yes\n",
    "* Can we assume we already have a linked list class that can be used for this problem?\n",
    "    * Yes"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Test Cases\n",
    "\n",
    "* Empty list(s)\n",
    "* Add values of different lengths\n",
    "    * Input 1: 6->5->None\n",
    "    * Input 2: 9->8->7\n",
    "    * Result: 5->4->8\n",
    "* Add values of same lengths\n",
    "    * Exercised from values of different lengths\n",
    "    * Done here for completeness"
   ]
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    "## Algorithm\n",
    "\n",
    "We could solve this with an iterative or a recursive algorithm, both are well suited for this exercise.  We'll use a recursive algorithm for practice with recursion.  Note this takes an extra space of O(m) where m is the recursion depth.\n",
    "\n",
    "* Base case:\n",
    "    * if first and second lists are NULL AND carry is zero\n",
    "        * Return NULL\n",
    "* Recursive case:\n",
    "    * value = carry\n",
    "    * value += first.data + second.data\n",
    "    * remainder = value % 10\n",
    "    * new_carry = 1 if value >= 10, else 0\n",
    "    * Create a node with the remainder\n",
    "    * node.next = self.add(first.next, second.next, new_carry)\n",
    "    * Return node\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: O(n), extra space for result and recursion depth\n",
    "\n",
    "Notes:\n",
    "* Careful with adding if the lists differ\n",
    "    * Only add if a node is not NULL\n",
    "    * Alternatively, we could add trailing zeroes to the smaller list"
   ]
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   "cell_type": "markdown",
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   "source": [
    "## Code"
   ]
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  {
   "cell_type": "code",
   "execution_count": 1,
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   "source": [
    "%run linked_list.py"
   ]
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   "source": [
    "class MyLinkedList(LinkedList):\n",
    "    def __add__(self, first_node, second_node, carry):\n",
    "        if type(carry) != int and carry < 0:\n",
    "            raise ValueError('Invalid int argument: carry')\n",
    "        if first_node is None and second_node is None and carry == 0:\n",
    "            return None\n",
    "        value = carry\n",
    "        value += first_node.data if first_node is not None else 0\n",
    "        value += second_node.data if second_node is not None else 0\n",
    "        remainder = value % 10\n",
    "        new_carry = 1 if value >= 10 else 0\n",
    "        node = Node(remainder)\n",
    "        node.next = self.__add__(first_node.next if first_node is not None else None, \n",
    "                                 second_node.next if first_node is not None else None, \n",
    "                                 new_carry)\n",
    "        return node\n",
    "\n",
    "    def add(self, first_list, second_list):\n",
    "        if first_list is None or second_list is None:\n",
    "            return None\n",
    "        head = self.__add__(first_list.head, second_list.head, 0)\n",
    "        return MyLinkedList(head)"
   ]
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   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "*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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     "text": [
      "Test: Empty list(s)\n",
      "Test: Add values of different lengths\n",
      "Test: Add values of same lengths\n",
      "Success: test_add\n"
     ]
    }
   ],
   "source": [
    "from nose.tools import assert_equal\n",
    "\n",
    "class Test(object):\n",
    "    def test_add(self):\n",
    "        print('Test: Empty list(s)')\n",
    "        assert_equal(MyLinkedList().add(None, None), None)\n",
    "        assert_equal(MyLinkedList().add(Node(5), None), None)\n",
    "        assert_equal(MyLinkedList().add(None, Node(10)), None)\n",
    "\n",
    "        print('Test: Add values of different lengths')\n",
    "        # Input 1: 6->5->None\n",
    "        # Input 2: 9->8->7\n",
    "        # Result: 5->4->8\n",
    "        first_list = MyLinkedList(Node(6))\n",
    "        first_list.append(5)\n",
    "        second_list = MyLinkedList(Node(9))\n",
    "        second_list.append(8)\n",
    "        second_list.append(7)\n",
    "        result = MyLinkedList().add(first_list, second_list)\n",
    "        assert_equal(result.get_all_data(), [5, 4, 8])\n",
    "\n",
    "        print('Test: Add values of same lengths')\n",
    "        # Input 1: 6->5->4\n",
    "        # Input 2: 9->8->7\n",
    "        # Result: 5->4->2->1\n",
    "        first_head = Node(6)\n",
    "        first_list = MyLinkedList(first_head)\n",
    "        first_list.append(5)\n",
    "        first_list.append(4)\n",
    "        second_head = Node(9)\n",
    "        second_list = MyLinkedList(second_head)\n",
    "        second_list.append(8)\n",
    "        second_list.append(7)\n",
    "        result = MyLinkedList().add(first_list, second_list)\n",
    "        assert_equal(result.get_all_data(), [5, 4, 2, 1])\n",
    "        \n",
    "        print('Success: test_add')\n",
    "\n",
    "if __name__ == '__main__':\n",
    "    test = Test()\n",
    "    test.test_add()"
   ]
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