Reworked notebook: Added more detail to constraints and test cases. Reworked algorithm discussion and unit test.

linked-lists/add-reverse.ipynb

@@ -13,22 +13,29 @@
    "source": [
     "## Problem: Add two numbers whose digits are stored in a linked list in reverse order.\n",
     "\n",
-    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
     "* [Test Cases](#Test-Cases)\n",
     "* [Algorithm](#Algorithm)\n",
-    "* [Code](#Code)"
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Clarifying Questions\n",
+    "## 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"
+    "    * 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"
    ]
   },
   {
@@ -55,10 +62,6 @@
     "\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",
-    "* Test for error cases\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\n",
     "* Base case:\n",
     "    * if first and second lists are NULL AND carry is zero\n",
     "        * Return NULL\n",
@@ -73,7 +76,12 @@
     "\n",
     "Complexity:\n",
     "* Time: O(n)\n",
-    "* Space: O(n), extra space for result and recursion depth"
+    "* 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"
    ]
   },
   {
@@ -85,7 +93,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 1,
    "metadata": {
     "collapsed": true
    },
@@ -96,7 +104,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 2,
    "metadata": {
     "collapsed": false
    },
@@ -114,7 +122,6 @@
     "        remainder = value % 10\n",
     "        new_carry = 1 if value >= 10 else 0\n",
     "        node = Node(remainder)\n",
-    "        #import pdb; pdb.set_trace()\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",
@@ -127,28 +134,61 @@
     "        return MyLinkedList(head)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "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.*"
+   ]
+  },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 3,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "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": [
-    "print('Add values of different lengths')\n",
+    "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_head = Node(6)\n",
+    "        first_list = MyLinkedList(Node(6))\n",
-    "first_list = MyLinkedList(first_head)\n",
     "        first_list.append(5)\n",
-    "second_head = Node(9)\n",
+    "        second_list = MyLinkedList(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",
-    "result.print_list()\n",
+    "        assert_equal(result.get_all_data(), [5, 4, 8])\n",
-    "print('Add values of same lengths')\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",
@@ -161,11 +201,13 @@
     "        second_list.append(8)\n",
     "        second_list.append(7)\n",
     "        result = MyLinkedList().add(first_list, second_list)\n",
-    "result.print_list()\n",
+    "        assert_equal(result.get_all_data(), [5, 4, 2, 1])\n",
-    "print('Empty list(s)')\n",
+    "        \n",
-    "result = MyLinkedList().add(None, None)\n",
+    "        print('Success: test_add')\n",
-    "result = MyLinkedList().add(first_head, None)\n",
+    "\n",
-    "result = MyLinkedList().add(None, second_head)"
+    "if __name__ == '__main__':\n",
+    "    test = Test()\n",
+    "    test.test_add()"
    ]
   }
  ],