Python Program To Check Whether The Length Of Given Linked List Is Even Or Odd

Given a linked list, the task is to make a function which checks whether the length of the linked list is even or odd.  
Examples:

Input : 1->2->3->4->NULL
Output : Even

Input : 1->2->3->4->5->NULL
Output : Odd

Method 1: Count the codes linearly
Traverse the entire Linked List and keep counting the number of nodes. As soon as the loop is finished, we can check if the count is even or odd. You may try it yourself.
Method 2: Stepping 2 nodes at a time
Approach:

1. Take a pointer and move that pointer two nodes at a time
2. At the end, if the pointer is NULL then length is Even, else Odd.

Python3

# Python program to check length
# of a given linklist
# Defining structure
class Node:
    def __init__(self, d):
        self.data = d
        self.next = None
        self.head = None

    # Function to check the length
    # of linklist
    def LinkedListLength(self):
        while (self.head != None and
               self.head.next != None):
            self.head = self.head.next.next

        if(self.head == None):
            return 0
        return 1

    # Push function
    def push(self, info):

    # Allocating node
        node = Node(info)

    # Next of new node to head
        node.next = (self.head)

    # head points to new node
        (self.head) = node

# Driver code
head = Node(0)

# Adding elements to Linked List
head.push(4)
head.push(5)
head.push(7)
head.push(2)
head.push(9)
head.push(6)
head.push(1)
head.push(2)
head.push(0)
head.push(5)
head.push(5)
check = head.LinkedListLength()

# Checking for length of
# linklist
if(check == 0):
    print("Even")
else:
    print("Odd")
# This code is contributed by Prerna saini

Output

Odd

Time Complexity: O(n) 
Space Complexity: O(1)

Method: Using recursion

Python3

class Node:

    def __init__(self, d):

        self.data = d

        self.next = None

        self.head = None

    # Function to check the length

    # of linklist

    def LinkedListLength(self, current=None):

        # if current is None:

            #current = self.head

        if current is None:

            return 0

        return 1 + self.LinkedListLength(current.next)

    # Push function

    def push(self, info):

        # Allocating node

        node = Node(info)

    # Next of new node to head

        node.next = (self.head)

    # head points to new node

        (self.head) = node


# Driver code
head = Node(0)


# Adding elements to Linked List

head.push(4)

head.push(5)

head.push(7)

head.push(2)

head.push(9)

head.push(6)

head.push(1)

head.push(2)

head.push(0)

head.push(5)


length = head.LinkedListLength()


# Checking for length of

# linklist

if length % 2 == 0:

    print("Even")

else:

    print("Odd")
 # This code is contributed by Vinay Pinjala.

Output

Even

Time Complexity: O(n)
Auxiliary Space: O(n)

Please refer complete article on Check whether the length of given linked list is Even or Odd for more details!

Method: Using Two pointer approach:

Python3

# Define the Node class to create nodes for the linked list
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
# Define the LinkedList class to create and manage linked lists
class LinkedList:
    def __init__(self):
        self.head = None
    # Function to determine the length of the linked list
    def length(self):
        slow_ptr = self.head # Initialize the slow pointer to the head of the list
        fast_ptr = self.head # Initialize the fast pointer to the head of the list
        # Move the slow pointer one step at a time
        # Move the fast pointer two steps at a time
        while fast_ptr is not None and fast_ptr.next is not None:
            slow_ptr = slow_ptr.next
            fast_ptr = fast_ptr.next.next
        # If the fast pointer reaches the end of the list, the length is even
        if fast_ptr is None:
            return "Even"
        # If the fast pointer reaches the last node of the list, the length is odd
        else:
            return "Odd"
# Initialize a linked list and add some nodes to it
linked_list = LinkedList()
linked_list.head = Node(1)
linked_list.head.next = Node(2)
linked_list.head.next.next = Node(3)
linked_list.head.next.next.next = Node(4)
# Determine the length of the linked list
length = linked_list.length()
print("Length:", length)
#This code is contributed by Jyothi pinjala.

Output

Length: Even

Time Complexity: O(n)
Auxiliary Space: O(n)