Add After
by Isai Damier

#=======================================================================
# 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 after node x
  #
  # Time Complexity of Solution:
  #   Best = O(1) Worst = O(n).
  #
  # Description:
  #
  # Technical Details: We simply need to find node x on the list
  #   and then attach the new node containing el to said node, which
  #   we represent as tmp. If x is not found, then el is not added.
  #   instead of having a traversal algorithm all the time, we treat the
  #   possibility of x being the tail of the list as a special case
  #   where the problem could be solved in O(1)
  #===================================================================== 
 import collections
class SinglyLinkedList( object ):

  def __init__( self ):
    self.head , self.tail = None, None

  def addAfter( self, el, n ):
    if None == self.tail or n.data == self.tail.data:
      self.addToTail( el )
    else:
      tmp = self.head
      while None != tmp and n.data != tmp.data:
        tmp = tmp.next
      if None != tmp:
        tmp.next = Node( el, tmp.next )

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 addAfter method, of class SinglyLinkedList.
  #=====================================================================
  def testAddAfter( 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.addAfter( value, linkedList.find( 7 ) )
    expected = [9, 4, 5, 2, 1, 12, 6, 7, 51, 4, 8, 3, 0, 16, 19, 11]
    self.assertEquals( expected, linkedList.toArray() )