Swap Elements
by Isai Damier, Android Engineer @ Google

#=======================================================================
# 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: Swap the given elements if they both exist. Return
  #   True for success, False for failure. If either of the inputs
  #   has duplicates, this function swaps the first occurrences only.
  #=====================================================================
 
 import collections
class SinglyLinkedList( object ):

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

  def swap( self, a, b ):
    # sometimes Python sucks: Should be able to declare and assign in if.
    A, B = self.find( a ), self.find( b )
    if None != A and None != B:
      A.data = b
      B.data = a
      return True
    return False

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 swap method, of class SinglyLinkedList.
  #=====================================================================
  def testSwap( 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] )

    linkedList.swap( 9, 11 )
    expected = [11, 4, 5, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 9]
    self.assertEquals( expected, linkedList.toArray() )
    linkedList.swap( 1, 7 )
    exp = [11, 4, 5, 2, 7, 12, 6, 1, 4, 8, 3, 0, 16, 19, 9]
    self.assertEquals( exp, linkedList.toArray() )