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    "## Problem: Find the kth to last element of a linked list\n",
    "\n",
    "* [Clarifying Questions](#Clarifying-Questions)\n",
    "* [Test Cases](#Test-Cases)\n",
    "* [Algorithm](#Algorithm)\n",
    "* [Code](#Code)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Clarifying Questions\n",
    "\n",
    "* Do we assume the linked list class already exists?\n",
    "    * No, create one\n",
    "* Is k an integer?\n",
    "    * Yes\n",
    "* If k = 0, does this return the last element?\n",
    "    * Yes\n",
    "* What happens if k is greater than or equal to the length of the linked list?\n",
    "    * Return None\n",
    "* Can you insert NULL values in the list?\n",
    "    * No\n",
    "* Can use additional data structures?\n",
    "    * No"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Test Cases\n",
    "\n",
    "* Empty list\n",
    "* k is not an integer\n",
    "* k is >= the length of the linked list\n",
    "* One element, k = 0\n",
    "* General case with many elements, k < length of linked list"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Algorithm\n",
    "\n",
    "* Check for edge cases above, returning None for errors\n",
    "* Setup two pointers, current and previous\n",
    "* Give current a headstart, incrementing it once for k = 1, twice for k = 2, etc\n",
    "* Increment both pointers until current reaches the end\n",
    "* Return the value of previous\n",
    "\n",
    "Complexity:\n",
    "* Time: O(n)\n",
    "* Space: In-place"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
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   "source": [
    "class Node(object):\n",
    "    def __init__(self, data):\n",
    "        self.data = data\n",
    "        self.next = None\n",
    "        \n",
    "class LinkedList(object):\n",
    "    def __init__(self, head):\n",
    "        self.head = head\n",
    "        \n",
    "    def __len__(self):\n",
    "        curr = self.head\n",
    "        counter = 0\n",
    "        while curr is not None:\n",
    "            counter += 1\n",
    "            curr = curr.next\n",
    "        return counter\n",
    "        \n",
    "    def kth_to_last_elem(self, k):\n",
    "        if self.head is None:\n",
    "            return\n",
    "        if type(k) != int:\n",
    "            return\n",
    "        if k >= len(self):\n",
    "            return\n",
    "        curr = self.head\n",
    "        prev = self.head\n",
    "        counter = 0\n",
    "        while counter < k:\n",
    "            curr = curr.next\n",
    "            counter += 1\n",
    "            if curr is None:\n",
    "                return\n",
    "        while curr.next is not None:\n",
    "            curr = curr.next\n",
    "            prev = prev.next\n",
    "        return prev.data\n",
    "        \n",
    "    def insert_to_front(self, data):\n",
    "        if data is None:\n",
    "            return\n",
    "        node = Node(data)\n",
    "        if self.head is None:\n",
    "            self.head = node\n",
    "        else:\n",
    "            node.next = self.head\n",
    "            self.head = node\n",
    "            \n",
    "# Empty list\n",
    "linked_list = LinkedList(None)\n",
    "print(linked_list.kth_to_last_elem(0))\n",
    "# k is not an integer\n",
    "print(linked_list.kth_to_last_elem('a'))\n",
    "# k is >= the length of the linked list\n",
    "print(linked_list.kth_to_last_elem(100))\n",
    "# One element, k = 0\n",
    "head = Node(2)\n",
    "linked_list = LinkedList(head)\n",
    "print(linked_list.kth_to_last_elem(0))\n",
    "# General case with many elements, k < length of linked list\n",
    "linked_list.insert_to_front(1)\n",
    "linked_list.insert_to_front(3)\n",
    "linked_list.insert_to_front(5)\n",
    "linked_list.insert_to_front(7)\n",
    "print(linked_list.kth_to_last_elem(2))"
   ]
  }
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