#=======================================================================
# Author: Isai Damier
# Title: Singly Linked List
# Project: geekviewpoint
# Package: datastructure
#
# Description: A LinkedList is a data structure that allows access
# to a collection of data using pointers/references. While an
# array can also be defined as above, LinkedLists and arrays differ
# in how they are stored in memory and in the operations they
# allow. Unlike an array that must be stored in a block of memory,
# the nodes of a LinkedList can be stored anywhere because each
# node has a reference to the node that succeeds it. Because the
# nodes are stored so loosely, inserting nodes into a LinkedList
# is easy; whereas in an array, all the succeeding elements must
# be shifted. Of course, insertion also means changing the size of
# the array, which means creating the entire array anew.
#
# Perhaps the greatest beauty of LinkedList is that it allows
# accessing an entire sequence of nodes using only one variable:
# a reference to the first node in the sequence.
#
# Countless operations can be performed on LinkedLists. Following
# are a few, ranging from the common to the very interesting.
#=======================================================================
#=====================================================================
# Statement:
# Retrieve the node that is k units from the tail.
#
# Time Complexity of Solution:
# Best = Worst = O(n).
#
# Technical Details: The imagery is to get a poll, k units long.
# Place the poll along the LinkedList so that one end
# (labeled X) of the poll is by the self.head node and one end
# (labeled Y) is by some other node called tmp. Now slide the
# poll along the LinkedList, until the end labeled Y reaches
# the self.tail. At this point, return the node by X.
#=====================================================================
import collections
class SinglyLinkedList( object ):
def __init__( self ):
self.head , self.tail = None, None
def findKthFromTail( self, k ):
if 0 > k:
return None
# count k units from the self.head.
tmp = self.head
count = 0
while count < k and None != tmp:
tmp = tmp.next
count += 1
# if the LinkedList does not contain k elements, return None
if count < k or None == tmp:
return None
# keeping tab on the kth element from tmp, slide tmp until
# tmp equals self.tail. Then return the kth element.
kth = self.head
while None != tmp.next:
tmp = tmp.next
kth = kth.next
return kth
class Node( object ):
def __init__( self, data, next = None ):
self.data = data
self.next = next
import unittest
from algorithms.SinglyLinkedList import SinglyLinkedList
import random
class Test( unittest.TestCase ):
#=====================================================================
# Test of findKthFromTail method, of class SinglyLinkedList.
#=====================================================================
def testFindKthFromTail( self ):
tape = [9, 4, 5, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 11]
linkedList = SinglyLinkedList()
for i in range( len( tape ) ):
linkedList.addToTail( tape[i] )
k = 0
last = len( tape ) - 1
self.assertEquals( tape[last - k], linkedList.findKthFromTail( k ).data )
k = last / 2
self.assertEquals( tape[last - k], linkedList.findKthFromTail( k ).data )
k = last
self.assertEquals( tape[last - k], linkedList.findKthFromTail( k ).data )
k = last + 21
self.assertEquals( None, linkedList.findKthFromTail( k ) )
k = -1
self.assertEquals( None, linkedList.findKthFromTail( k ) )
