Reverse in Pair
by Isai Damier, Android Engineer @ Google

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#=======================================================================
# 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: Reverse each pair of nodes, such that abcdefg
  #   becomes badcfeg.
  #=====================================================================
  
 import collections
class SinglyLinkedList( object ):
 
  def __init__( self ):
    self.head , self.tail = None, None
 
  def reverseInPairs( self ) :
    t = self.head
    while None != t and None != t.next:
      self.swapData( t, t.next )
      t = t.next.next
 
  def swapData( self, a, b ):
    tmp = a.data
    a.data = b.data
    b.data = tmp
 
 
class Node( object ):
 
  def __init__( self, data, next = None ):
    self.data = data
    self.next = next