#=======================================================================
# 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*log n).
#
# Description: Sort the elements in this linked list in ascending
# order.
#
# Technical Details: Sorting a LinkedList in place is very costly:
# in the order of O(n^2# log(n)) with a good algorithm.
# Therefore it makes sense to create an array from the list,
# sort the array, and then empty the array into the list by
# index. It's kind of like: unbutton a shirt; move the buttons
# around; then button the shirt anew. (I think it's a good analogy.)
#=====================================================================
import collections
class SinglyLinkedList( object ):
def __init__( self ):
self.head , self.tail = None, None
def sort( self ) :
if 2 > self.size():
return
A = [0] * self.size()
# unfasten the buttons (i.e. unbutton the shirt)
x = 0
t = self.head
while None != t:
A[x] = t.data # list fills array
x += 1
t = t.next
self._mergesort( A, 0, len( A ) - 1 )
# refasten the buttons (i.e. button the shirt anew)
x = 0
t = self.head
while None != t:
t.data = A[x] # array fills list
x += 1
t = t.next
def _mergesort( self, aList, first, last ):
# break problem into smaller structurally identical pieces
mid = ( first + last ) / 2
if first < last:
self._mergesort( aList, first, mid )
self._mergesort( aList, mid + 1, last )
# merge solved pieces to get solution to original problem
a, f, l = 0, first, mid + 1
tmp = [None] * ( last - first + 1 )
while f <= mid and l <= last:
if aList[f] < aList[l] :
tmp[a] = aList[f]
f += 1
else:
tmp[a] = aList[l]
l += 1
a += 1
if f <= mid :
tmp[a:] = aList[f:mid + 1]
if l <= last:
tmp[a:] = aList[l:last + 1]
a = 0
while first <= last:
aList[first] = tmp[a]
first += 1
a += 1
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 sort method, of class SinglyLinkedList.
#=====================================================================
def testSort( 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() )
tape.sort()
self.assertNotEquals( tape, linkedList.toArray() )
linkedList.sort()
self.assertEquals( tape, linkedList.toArray() )
