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Added notebook solving the following: Determine if a linked list is a palindrome.
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Donne Martin
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@ -28,6 +28,8 @@ Continually updated IPython Notebooks containing coding problems and solutions (
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* [Partition a linked list around a given value](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/partition.ipynb)
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* [Add two numbers whose digits are stored in a linked list in reverse order](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/add-reverse.ipynb)
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* [Find the start of a linked list loop](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/find-loop-start.ipynb)
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* [Determine if a linked list is a palindrome](http://nbviewer.ipython.org/github/donnemartin/algorithms-data-structures/blob/master/linked-lists/palindrome.ipynb)
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## Stacks and Queues
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166
linked-lists/palindrome.ipynb
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166
linked-lists/palindrome.ipynb
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@ -0,0 +1,166 @@
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Problem: Determine if a linked list is a palindrome.\n",
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"\n",
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"* [Clarifying Questions](#Clarifying-Questions)\n",
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"* [Test Cases](#Test-Cases)\n",
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"* [Algorithm](#Algorithm)\n",
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"* [Code](#Code)\n",
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"* [Pythonic-Code](#Pythonic-Code)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Clarifying Questions\n",
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"\n",
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"* Is a single character or number a palindrome?\n",
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" * No"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Test Cases\n",
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"\n",
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"* Empty list\n",
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"* Single element list\n",
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"* Two element list, not a palindrome\n",
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"* Three element list, not a palindrome\n",
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"* General case: Palindrome with even length\n",
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"* General case: Palindrome with odd length"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Algorithm\n",
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"\n",
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"* Reverse the linked list\n",
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"* Compare the reversed list with the original list\n",
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" * Only need to compare the first half\n",
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"\n",
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"Complexity:\n",
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"* Time: O(1)\n",
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"* Space: O(n)\n",
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"\n",
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"Note:\n",
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"* We could also do this iteratively, using a stack to effectively reverse the first half of the string."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Code"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": true
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},
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"outputs": [],
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"source": [
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"%run linked_list.py"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"class MyLinkedList(LinkedList):\n",
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" def is_palindrome(self):\n",
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" if self.head is None or self.head.next is None:\n",
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" return False\n",
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" curr = self.head\n",
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" reversed_list = MyLinkedList()\n",
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" length = 0\n",
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" while curr is not None:\n",
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" reversed_list.insert_to_front(curr.data)\n",
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" length += 1\n",
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" curr = curr.next\n",
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" iterations_to_compare_half = length / 2\n",
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" curr = self.head\n",
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" curr_reversed = reversed_list.head\n",
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" for _ in xrange(0, iterations_to_compare_half):\n",
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" if curr.data != curr_reversed.data:\n",
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" return False\n",
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" curr = curr.next\n",
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" curr_reversed = curr_reversed.next\n",
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" return True"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {
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"collapsed": false
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},
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"outputs": [],
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"source": [
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"print('Empty list')\n",
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"linked_list = MyLinkedList()\n",
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"print(linked_list.is_palindrome())\n",
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"print('Single element list')\n",
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"head = Node(1)\n",
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"linked_list = MyLinkedList(head)\n",
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"print(linked_list.is_palindrome())\n",
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"print('Two element list, not a palindrome')\n",
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"linked_list.append(2)\n",
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"print(linked_list.is_palindrome())\n",
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"print('Three element list, not a palindrome')\n",
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"linked_list.append(3)\n",
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"print(linked_list.is_palindrome())\n",
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"print('General case: Palindrome with even length')\n",
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"head = Node('a')\n",
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"linked_list = MyLinkedList(head)\n",
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"linked_list.append('b')\n",
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"linked_list.append('b')\n",
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"linked_list.append('a')\n",
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"print(linked_list.is_palindrome())\n",
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"print('General case: Palindrome with odd length')\n",
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"head = Node(1)\n",
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"linked_list = MyLinkedList(head)\n",
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"linked_list.append(2)\n",
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"linked_list.append(3)\n",
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"linked_list.append(2)\n",
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"linked_list.append(1)\n",
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"print(linked_list.is_palindrome())"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 2",
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"language": "python",
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"name": "python2"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 2
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython2",
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"version": "2.7.9"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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