Python Program To Check If A Singly Linked List Is Palindrome

# Python3 program to check if linked
# list is palindrome using stack
class Node:
    def __init__(self, data):        
        self.data = data
        self.ptr = None
        
# Function to check if the linked list
# is palindrome or not
def ispalindrome(head):
    
    # Temp pointer
    slow = head

    # Declare a stack
    stack = []
    
    ispalin = True

    # Push all elements of the list
    # to the stack
    while slow != None:
        stack.append(slow.data)
        
        # Move ahead
        slow = slow.ptr

    # Iterate in the list again and
    # check by popping from the stack
    while head != None:

        # Get the top most element
        i = stack.pop()
        
        # Check if data is not
        # same as popped element
        if head.data == i:
            ispalin = True
        else:
            ispalin = False
            break

        # Move ahead
        head = head.ptr
        
    return ispalin

# Driver Code

# Addition of linked list
one = Node(1)
two = Node(2)
three = Node(3)
four = Node(4)
five = Node(3)
six = Node(2)
seven = Node(1)

# Initialize the next pointer
# of every current pointer
one.ptr = two
two.ptr = three
three.ptr = four
four.ptr = five
five.ptr = six
six.ptr = seven
seven.ptr = None

# Call function to check 
# palindrome or not
result = ispalindrome(one)

print("isPalindrome:", result)

# This code is contributed by Nishtha Goel

# Python program to check if
# linked list is palindrome

# Node class
class Node:

    # Constructor to initialize
    # the node object
    def __init__(self, data):        
        self.data = data
        self.next = None

class LinkedList:

    # Function to initialize head
    def __init__(self):        
        self.head = None

    # Function to check if given
    # linked list is palindrome or not
    def isPalindrome(self, head):        
        slow_ptr = head
        fast_ptr = head
        prev_of_slow_ptr = head
        
        # To handle odd size list
        midnode = None
        
        # Initialize result
        res = True  
        
        if (head != None and 
            head.next != None):
            
            # Get the middle of the list. 
            # Move slow_ptr by 1 and 
            # fast_ptrr by 2, slow_ptr 
            # will have the middle node
            while (fast_ptr != None and 
                   fast_ptr.next != None):
                      
                # We need previous of the slow_ptr 
                # for linked lists  with odd 
                # elements
                fast_ptr = fast_ptr.next.next
                prev_of_slow_ptr = slow_ptr
                slow_ptr = slow_ptr.next
                
            # fast_ptr would become NULL when 
            # there are even elements in the 
            # list and not NULL for odd elements. 
            # We need to skip the middle node for 
            # odd case and store it somewhere so 
            # that we can restore the original list
            if (fast_ptr != None):
                midnode = slow_ptr
                slow_ptr = slow_ptr.next
                
            # Now reverse the second half 
            # and compare it with the first half
            second_half = slow_ptr
            
            # NULL terminate first half
            prev_of_slow_ptr.next = None 
            
            # Reverse the second half
            second_half = self.reverse(second_half) 
            
            # Compare
            res = self.compareLists(head, second_half)  
            
            # Construct the original list back
            # Reverse the second half again
            second_half = self.reverse(second_half)
            
            if (midnode != None):
                
                # If there was a mid node (odd size
                # case) which was not part of either
                # first half or second half.
                prev_of_slow_ptr.next = midnode
                midnode.next = second_half
            else:
                prev_of_slow_ptr.next = second_half
        return res
    
    # Function to reverse the linked list 
    # Note that this function may change 
    # the head
    def reverse(self, second_half):
        
        prev = None
        current = second_half
        next = None
        
        while current != None:
            next = current.next
            current.next = prev
            prev = current
            current = next
            
        second_half = prev
        return second_half

    # Function to check if two input 
    # lists have same data
    def compareLists(self, head1, head2):
        
        temp1 = head1
        temp2 = head2
        
        while (temp1 and temp2):
            if (temp1.data == temp2.data):
                temp1 = temp1.next
                temp2 = temp2.next
            else:
                return 0
                
        # Both are empty return 1
        if (temp1 == None and temp2 == None):
            return 1
            
        # Will reach here when one is NULL
        # and other is not
        return 0
    
    # Function to insert a new node
    # at the beginning
    def push(self, new_data):
        
        # Allocate the Node &
        # Put in the data
        new_node = Node(new_data)
        
        # Link the old list of the new one
        new_node.next = self.head
        
        # Move the head to point to the 
        # new Node
        self.head = new_node

    # A utility function to print
    # a given linked list 
    def printList(self):
        
        temp = self.head
        
        while(temp):
            print(temp.data, end = "->")
            temp = temp.next
            
        print("NULL")

# Driver code
if __name__ == '__main__':    
    l = LinkedList()
    s = ['a', 'b', 'a', 
         'c', 'a', 'b', 'a']
    
    for i in range(7):
        l.push(s[i])
        l.printList()
        
        if (l.isPalindrome(l.head) != False):
            print("Is Palindrome")
        else:
            print("Not Palindrome")
        print()

# This code is contributed by MuskanKalra1