Reverse
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.
#=======================================================================
  #=====================================================================
  # Time Complexity of Solution:
  #   O(n).
  #
  # Description: Reverse this LinkedList.
  #
  # Technical Details: This algorithm reverses this singly linked
  #   list in place, in O(n). The function uses three pointers to
  #   walk the list and reverse link direction between each pair
  #   of nodes.
  #
  #    BTY: reversing a P-shaped LinkedList still results in a
  #     P-shaped LinkedList with the same self.head and linear section;
  #     only the direction of the circular portion is reversed.
  #=====================================================================
 
 import collections
class SinglyLinkedList( object ):

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

  def reverse( self ) :
    if None == self.head or None == self.head.next:
      return

    a = self.head
    b = a.next
    c = b.next

    # swaps
    a.next = None
    b.next = a;
    a = b;
    while None != c:
      b = c
      c = c.next
      b.next = a
      a = b

    self.head = b


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 reverse method, of class SinglyLinkedList.
  #=====================================================================
  def testReverse( 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] )

    self.assertEquals( tape, linkedList.toArray() )
    linkedList.reverse()
    ndx = len( tape )
    n = linkedList.head
    while None != n :
      ndx -= 1
      self.assertEquals( tape[ndx], n.data )
      n = n.next