#=======================================================================
# 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:
# Add element el before node x
#
# Time Complexity of Solution:
# Best = O(1); Worst = O(n).
#
# Technical Details: The idea here is to insert element el between
# node x and it's parent. So if node x is the head of the list,
# el is added before node x and thus becomes the new head else,
# we traverse the list looking for node x and it's parent so to add
# el between x and the parent. If node x is not on the list, then
# element el is added as the tail of the list.
#=====================================================================
import collections
class SinglyLinkedList( object ):
def __init__( self ):
self.head , self.tail = None, None
def addBefore( self, el, n ):
if None == self.head or n.data == self.head.data :
self.addToHead( el )
else:
parent = None
curr = self.head
while None != curr and curr.data != n.data:
parent = curr
curr = curr.next
if None != curr:
parent.next = Node( el, n )
else:
self.addToTail( el )
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 addBefore method, of class SinglyLinkedList.
#=====================================================================
def testAddBefore( 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] )
value = 51
linkedList.addBefore( value, linkedList.find( 7 ) )
expected = [9, 4, 5, 2, 1, 12, 6, 51, 7, 4, 8, 3, 0, 16, 19, 11]
self.assertEquals( expected, linkedList.toArray() )
