Delete from Tail
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:
  #   Retrieve and remove the tail of this LinkedList.
  #
  # Time Complexity of Solution:
  #   O(n).
  #
  # Technical Details: Because the element preceding the self.tail must
  #   now become the new self.tail, this is a O(n) operation.
  #
  #=====================================================================
 
 import collections
class SinglyLinkedList( object ):

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

  def deleteFromTail( self ) :
    delt = self.tail
    if self.head == self.tail:
      self.head = self.tail = None # same as self.clear()
    else:
      p = self.head
      while p.next != self.tail:
        p = p.next
      p.next = None
      self.tail = p

    return delt


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 deleteFromTail method, of class SinglyLinkedList.
  #=====================================================================
  def testDeleteFromTail( self ):
    tape = [9, 4, 5, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 11]
    linkedList = SinglyLinkedList()
    self.assertIsNone( linkedList.deleteFromHead() )
    for i in range( len( tape ) ):
      linkedList.addToTail( tape[i] )

    ndx = len( tape ) - 1
    while ndx >= 0:
      self.assertEquals( tape[ndx], linkedList.deleteFromTail().data )
      ndx -= 1