Python Program For Sorting A Linked List That Is Sorted Alternating Ascending And Descending Orders

# Python program to sort a linked list 
# that is alternatively sorted in 
# increasing and decreasing order
class LinkedList(object):
    def __init__(self):
        self.head = None

    # Linked list Node
    class Node(object):
        def __init__(self, d):
            self.data = d
            self.next = None

    def newNode(self, key):
        return self.Node(key)

    # This is the main function that sorts
    # the linked list.
    def sort(self):
        # Create two dummy nodes and 
        # initialize as
        # heads of linked lists
        Ahead = self.Node(0)
        Dhead = self.Node(0)

        # Split the list into lists
        self.splitList(Ahead, Dhead)
        Ahead = Ahead.next
        Dhead = Dhead.next

        # Reverse the descending list
        Dhead = self.reverseList(Dhead)

        # Merge the 2 linked lists
        self.head = self.mergeList(Ahead, 
                                   Dhead)

    # Function to reverse the linked list
    def reverseList(self, Dhead):
        current = Dhead
        prev = None
        while current != None:
            self._next = current.next
            current.next = prev
            prev = current
            current = self._next
        Dhead = prev
        return Dhead

    # Function to print linked list
    def printList(self):
        temp = self.head
        while temp != None:
            print temp.data,
            temp = temp.next
        print ''

    # A utility function to merge two 
    # sorted linked lists
    def mergeList(self, head1, head2):
        # Base cases
        if head1 == None:
            return head2
        if head2 == None:
            return head1
        temp = None
        if head1.data < head2.data:
            temp = head1
            head1.next = self.mergeList(head1.next, 
                                        head2)
        else:
            temp = head2
            head2.next = self.mergeList(head1, 
                                        head2.next)
        return temp

    # This function alternatively splits a 
    # linked list with head as head into two:
    # For example, 10->20->30->15->40->7 is 
    # splitted into 10->30->40 and 20->15->7
    # "Ahead" is reference to head of ascending 
    # linked list
    # "Dhead" is reference to head of descending 
    # linked list
    def splitList(self, Ahead, Dhead):
        ascn = Ahead
        dscn = Dhead
        curr = self.head

        # Link alternate nodes
        while curr != None:

            # Link alternate nodes in ascending 
            # order
            ascn.next = curr
            ascn = ascn.next
            curr = curr.next
            if curr != None:
                dscn.next = curr
                dscn = dscn.next
                curr = curr.next
        ascn.next = None
        dscn.next = None

# Driver code
llist = LinkedList()
llist.head = llist.newNode(10)
llist.head.next = llist.newNode(40)
llist.head.next.next = 
llist.newNode(53)
llist.head.next.next.next = 
llist.newNode(30)
llist.head.next.next.next.next = 
llist.newNode(67)
llist.head.next.next.next.next.next = 
llist.newNode(12)
llist.head.next.next.next.next.next.next = 
llist.newNode(89)

print 'Given linked list'
llist.printList()

llist.sort()

print 'Sorted linked list'
llist.printList()
# This code is contributed by BHAVYA JAIN