Python Program to Check whether a Singly Linked List is a Palindrome

This is a Python program to check whether a singly linked list is a palindrome.

The program creates a linked list using data items input from the user and determines whether it is a palindrome.

1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variables head and last_node.
3. The variable head points to the first element in the linked list while last_node points to the last.
4. Define methods append and display inside the class LinkedList to append data and display the linked list respectively.
5. Define method get_prev_node which takes a reference node as argument and returns the node before it.
6. Define a function is_palindrome which returns True if the linked list passed to it is a palindrome.
7. The function is_palindrome iterates through the linked list from the start and the last node towards the middle to check if the list is a palindrome.
8. Create an instance of LinkedList, append data to it and determine whether it is a palindrome.

Here is the source code of a Python program to check whether a singly linked list is a palindrome.

class Node:
    def __init__(self, data):
       self.data = data
       self.next = None
 
 
class LinkedList:
    def __init__(self):
        self.head = None
        self.last_node = None
 
    def append(self, data):
        if self.last_node is None:
            self.head = Node(data)
            self.last_node = self.head
        else:
            self.last_node.next = Node(data)
            self.last_node = self.last_node.next
 
    def get_prev_node(self, ref_node):
        current = self.head
        while (current and current.next != ref_node):
            current = current.next
        return current
 
 
def is_palindrome(llist):
    start = llist.head
    end = llist.last_node
    while (start != end and end.next != start):
        if start.data != end.data:
            return False
        start = start.next
        end = llist.get_prev_node(end)
    return True
 
 
a_llist = LinkedList()
 
data_list = input('Please enter the elements in the linked list: ').split()
for data in data_list:
    a_llist.append(int(data))
 
if is_palindrome(a_llist):
    print('The linked list is palindromic.')
else:
    print('The linked list is not palindromic.')

1. An instance of LinkedList is created.
2. The user is prompted to enter the data items for the list.
3. The function is_palindrome is called with the list as argument to determine whether it is a palindrome.
4. The result is then displayed.

Case 1:
Please enter the elements in the linked list: 7 8 1 8 7
The linked lists is palindromic.
 
Case 2:
Please enter the elements in the linked list: 1 2 3 3 2 1
The linked list is palindromic.
 
Case 3:
Please enter the elements in the linked list: 1 4 5 4 5 1
The linked list is not palindromic.

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