Python Program For Sorting A Linked List Of 0s, 1s And 2s By Changing Links

# Python3 Program to sort a linked list 
# 0s, 1s or 2s by changing links
import math

# Link list node 
class Node: 
    def __init__(self, data): 
        self.data = data 
        self.next = None

#Node* newNode( data)

# Sort a linked list of 0s, 1s 
# and 2s by changing pointers.
def sortList(head):
    if (head == None or 
        head.next == None):
        return head

    # Create three dummy nodes to point 
    # to beginning of three linked lists. 
    # These dummy nodes are created to 
    # avoid many None checks.
    zeroD = Node(0)
    oneD = Node(0)
    twoD = Node(0)

    # Initialize current pointers for 
    # three lists and whole list.
    zero = zeroD 
    one = oneD 
    two = twoD

    # Traverse list
    curr = head
    while (curr):
        if (curr.data == 0):
            zero.next = curr
            zero = zero.next
            curr = curr.next
        elif(curr.data == 1):
            one.next = curr
            one = one.next
            curr = curr.next
        else:
            two.next = curr
            two = two.next
            curr = curr.next
        
    # Attach three lists
    zero.next = (oneD.next) if (oneD.next) 
                            else (twoD.next)
    one.next = twoD.next
    two.next = None

    # Updated head
    head = zeroD.next

    # Delete dummy nodes
    return head

# Function to create and return 
# a node
def newNode(data):
    
    # Allocating space
    newNode = Node(data)

    # Inserting the required data
    newNode.data = data
    newNode.next = None
    return newNode

# Function to print linked list 
def prList(node):
    while (node != None):
        print(node.data, end = " ")
        node = node.next
    
# Driver Code
if __name__=='__main__':
    
    # Creating the list 1.2.4.5
    head = newNode(1)
    head.next = newNode(2)
    head.next.next = newNode(0)
    head.next.next.next = newNode(1)

    print("Linked List Before Sorting")
    prList(head)

    head = sortList(head)

    print("Linked List After Sorting")
    prList(head)
# This code is contributed by Srathore