#=======================================================================
# 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.
#=======================================================================
#=====================================================================
# Time Complexity of Solution:
# O(n).
#
# Description: Find the node (intersection) where the two given
# LinkedList coalesce.
#
# Technical Details: The idea is that two linked lists form a Y-shaped
# structure. The structure may be degenerated. As shown below, the use
# of a hashmap greatly reduces the complexity of the problem.
#
#
# To keep things simple, we consider t2 and t2 equal iff they both
# point to the same memory location. A broader definition of
# equality: Two nodes t2 and t1 are equal if they and their respective
# sublists are equal.
#=====================================================================
import collections
class SinglyLinkedList( object ):
def __init__( self ):
self.head , self.tail = None, None
@classmethod
def findIntersection( self, head1, head2 ):
intersect = {}
t = head1
while None != t:
intersect[t] = True
t = t.next
# first duplicate is intersection
t = head2
while None != t:
if None != intersect.get( t ):
return t
t = t.next
return None
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 findIntersection method, of class SinglyLinkedList.
#=====================================================================
def testFindIntersection( self ):
in_1 = [29, 14, 35, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 11]
in_2 = [99, 78, 8, 3, 23]
list_1 = SinglyLinkedList()
list_2 = SinglyLinkedList()
for i in range( len( in_1 ) ):
list_1.addToTail( in_1[i] )
self.assertEquals( len( in_1 ), list_1.size() )
for i in range( len( in_2 ) ):
list_2.addToTail( in_2[i] )
self.assertEquals( len( in_2 ), list_2.size() )
# set intersection: list_1 and list_2 form a Y at element 6
exp = 6
found = list_1.find( exp )
self.assertIsNotNone( found )
list_2.addAllToTail( found )
self.assertEquals( 14, list_2.size() )
n = SinglyLinkedList.findIntersection( list_1.head, list_2.head )
self.assertEquals( exp, n.data )