Add Before
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 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() )