Added notebook solving the following: Find the kth to last element of a linked list.

README.md

@@ -16,6 +16,7 @@ Continually updated IPython Notebooks containing algorithms and data structures.
 
 * [Linked list with insert, find, delete, and print methods](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/linked-list.ipynb)
 * [Remove duplicates from a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/remove-duplicates.ipynb)
+* [Find the kth to last element of a linked list](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/kth-to-last-elem.ipynb)
 
 ## License
 

linked-lists/kth-to-last-elem.ipynb

@@ -0,0 +1,168 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## 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"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "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))"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.9"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}