Python Program For Merge Sort For Doubly Linked List

# Program for merge sort on doubly linked list
# A node of the doubly 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 "
Backward 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)