Added Merge Sort for Doubly Linked List

Python/sorts/merge_sort_for_doubly_linked_list.py

@@ -0,0 +1,118 @@
+# Program for merge sort on doubly linked list
+
+# A node of the doublly linked list
+class Node:
+
+	# Constructor to create a new node
+	def __init__(self, data):
+		self.data = data
+		self.next = None
+		self.prev = None
+
+class DoublyLinkedList:
+
+	# Constructor for empty Doubly Linked List
+	def __init__(self):
+		self.head = None
+
+	# Function to merge two linked list
+	def merge(self, first, second):
+
+		# If first linked list is empty
+		if first is None:
+			return second
+
+		# If second linked list is empty
+		if second is None:
+			return first
+
+		# Pick the smaller value
+		if first.data < second.data:
+			first.next = self.merge(first.next, second)
+			first.next.prev = first
+			first.prev = None
+			return first
+		else:
+			second.next = self.merge(first, second.next)
+			second.next.prev = second
+			second.prev = None
+			return second
+
+	# Function to do merge sort
+	def mergeSort(self, tempHead):
+		if tempHead is None:
+			return tempHead
+		if tempHead.next is None:
+			return tempHead
+
+		second = self.split(tempHead)
+
+		# Recur for left and right halves
+		tempHead = self.mergeSort(tempHead)
+		second = self.mergeSort(second)
+
+		# Merge the two sorted halves
+		return self.merge(tempHead, second)
+
+	# Split the doubly linked list (DLL) into two DLLs
+	# of half sizes
+	def split(self, tempHead):
+		fast = slow = tempHead
+		while(True):
+			if fast.next is None:
+				break
+			if fast.next.next is None:
+				break
+			fast = fast.next.next
+			slow = slow.next
+
+		temp = slow.next
+		slow.next = None
+		return temp
+
+
+	# Given a reference to the head of a list and an
+	# integer,inserts a new node on the front of list
+	def push(self, new_data):
+
+		# 1. Allocates node
+		# 2. Put the data in it
+		new_node = Node(new_data)
+
+		# 3. Make next of new node as head and
+		# previous as None (already None)
+		new_node.next = self.head
+
+		# 4. change prev of head node to new_node
+		if self.head is not None:
+			self.head.prev = new_node
+
+		# 5. move the head to point to the new node
+		self.head = new_node
+
+
+	def printList(self, node):
+		temp = node
+		print "Forward Traversal using next pointer"
+		while(node is not None):
+			print node.data,
+			temp = node
+			node = node.next
+		print "\nBackward Traversal using prev pointer"
+		while(temp):
+			print temp.data,
+			temp = temp.prev
+
+# Driver program to test the above functions
+dll = DoublyLinkedList()
+dll.push(5)
+dll.push(20);
+dll.push(4);
+dll.push(3);
+dll.push(30)
+dll.push(10);
+dll.head = dll.mergeSort(dll.head)
+print "Linked List after sorting"
+dll.printList(dll.head)
+
+# This code is contributed by Nikhil Kumar Singh(nickzuck_007)