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Function to check if a singly linked list is palindrome

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  • Difficulty Level : Medium
  • Last Updated : 24 Jun, 2022
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Given a singly linked list of characters, write a function that returns true if the given list is a palindrome, else false.

Palindrome Linked List

METHOD 1 (Use a Stack) 

  • A simple solution is to use a stack of list nodes. This mainly involves three steps.
  • Traverse the given list from head to tail and push every visited node to stack.
  • Traverse the list again. For every visited node, pop a node from the stack and compare data of popped node with the currently visited node.
  • If all nodes matched, then return true, else false.

Complete Interview Preparation - GFG

Below image is a dry run of the above approach: 

Below is the implementation of the above approach : 

C++




#include<bits/stdc++.h>
using namespace std; 
  
class Node {
public:
        int data;
        Node(int d){
            data = d;
        }
        Node *ptr;
};
  
// Function to check if the linked list 
// is palindrome or not 
bool isPalin(Node* head){
          
        // Temp pointer 
        Node* slow= head;
  
        // Declare a stack 
        stack <int> s;
   
  
        // Push all elements of the list 
        // to the stack 
        while(slow != NULL){
                s.push(slow->data);
  
                // Move ahead 
                slow = slow->ptr;
        }
  
        // Iterate in the list again and 
        // check by popping from the stack
        while(head != NULL ){
              
            // Get the top most element 
             int i=s.top();
  
             // Pop the element 
             s.pop();
  
             // Check if data is not
             // same as popped element 
            if(head -> data != i){
                return false;
            }
  
            // Move ahead 
           head=head->ptr;
        }
  
return true;
}
  
// Driver Code 
int main(){
  
    // Addition of linked list 
    Node one =  Node(1);
    Node two = Node(2);
    Node three = Node(3);
    Node four = Node(2);
    Node five = Node(1);
  
    // Initialize the next pointer
    // of every current pointer 
    five.ptr = NULL;
    one.ptr = &two;
    two.ptr = &three;
    three.ptr = &four;
    four.ptr = &five;
    Node* temp = &one;
  
      
    // Call function to check palindrome or not 
    int result = isPalin(&one);
    
    if(result == 1)
            cout<<"isPalindrome is true\n";
    else
        cout<<"isPalindrome is true\n";
  
return 0;
}
  
// This code has been contributed by Striver 


Java




/* Java program to check if linked list is palindrome recursively */
import java.util.*;
  
class linkedList {
    public static void main(String args[])
    {
        Node one = new Node(1);
        Node two = new Node(2);
        Node three = new Node(3);
        Node four = new Node(4);
        Node five = new Node(3);
        Node six = new Node(2);
        Node seven = new Node(1);
        one.ptr = two;
        two.ptr = three;
        three.ptr = four;
        four.ptr = five;
        five.ptr = six;
        six.ptr = seven;
        boolean condition = isPalindrome(one);
        System.out.println("isPalidrome :" + condition);
    }
    static boolean isPalindrome(Node head)
    {
  
        Node slow = head;
        boolean ispalin = true;
        Stack<Integer> stack = new Stack<Integer>();
  
        while (slow != null) {
            stack.push(slow.data);
            slow = slow.ptr;
        }
  
        while (head != null) {
  
            int i = stack.pop();
            if (head.data == i) {
                ispalin = true;
            }
            else {
                ispalin = false;
                break;
            }
            head = head.ptr;
        }
        return ispalin;
    }
}
  
class Node {
    int data;
    Node ptr;
    Node(int d)
    {
        ptr = null;
        data = d;
    }
}


Python3




# 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


C#




// C# program to check if linked list
// is palindrome recursively 
using System;
using System.Collections.Generic;
  
class linkedList{
      
// Driver code
public static void Main(String []args)
{
    Node one = new Node(1);
    Node two = new Node(2);
    Node three = new Node(3);
    Node four = new Node(4);
    Node five = new Node(3);
    Node six = new Node(2);
    Node seven = new Node(1);
      
    one.ptr = two;
    two.ptr = three;
    three.ptr = four;
    four.ptr = five;
    five.ptr = six;
    six.ptr = seven;
      
    bool condition = isPalindrome(one);
    Console.WriteLine("isPalidrome :" + condition);
}
  
static bool isPalindrome(Node head)
{
    Node slow = head;
    bool ispalin = true;
    Stack<int> stack = new Stack<int>();
  
    while (slow != null)
    {
        stack.Push(slow.data);
        slow = slow.ptr;
    }
  
    while (head != null)
    {
        int i = stack.Pop();
        if (head.data == i) 
        {
            ispalin = true;
        }
        else
        {
            ispalin = false;
            break;
        }
        head = head.ptr;
    }
    return ispalin;
}
}
  
class Node 
{
    public int data;
    public Node ptr;
    public Node(int d)
    {
        ptr = null;
        data = d;
    }
}
  
// This code is contributed by amal kumar choubey


Javascript




<script>
  
/* JavaScript program to check if
linked list is palindrome recursively */
  
    class Node {
        constructor(val) {
            this.data = val;
            this.ptr = null;
        }
    }
      
var one = new Node(1);
var two = new Node(2);
var three = new Node(3);
var four = new Node(4);
var five = new Node(3);
var six = new Node(2);
var seven = new Node(1);
        one.ptr = two;
        two.ptr = three;
        three.ptr = four;
        four.ptr = five;
        five.ptr = six;
        six.ptr = seven;
        var condition = isPalindrome(one);
        document.write("isPalidrome: " + condition);
      
  
    function isPalindrome(head) {
  
var slow = head;
        var ispalin = true;
        var stack = [];
  
        while (slow != null) {
            stack.push(slow.data);
            slow = slow.ptr;
        }
  
        while (head != null) {
  
            var i = stack.pop();
            if (head.data == i) {
                ispalin = true;
            } else {
                ispalin = false;
                break;
            }
            head = head.ptr;
        }
        return ispalin;
    }
  
  
// This code is contributed by todaysgaurav
  
</script>


Output 

 isPalindrome: true

Time complexity: O(n)

Auxiliary Space: O (n) since we are using an auxiliary stack

METHOD 2 (By reversing the list) 
This method takes O(n) time and O(1) extra space. 
1) Get the middle of the linked list. 
2) Reverse the second half of the linked list. 
3) Check if the first half and second half are identical. 
4) Construct the original linked list by reversing the second half again and attaching it back to the first half

To divide the list into two halves, method 2 of this post is used. 

When a number of nodes are even, the first and second half contain exactly half nodes. The challenging thing in this method is to handle the case when the number of nodes is odd. We don’t want the middle node as part of the lists as we are going to compare them for equality. For odd cases, we use a separate variable ‘midnode’. 

C++




// C++ program to check if a linked list is palindrome 
#include <bits/stdc++.h>
using namespace std;
  
// Link list node 
struct Node 
    char data; 
    struct Node* next; 
}; 
  
void reverse(struct Node**); 
bool compareLists(struct Node*, struct Node*); 
  
// Function to check if given linked list is 
// palindrome or not 
bool isPalindrome(struct Node* head) 
    struct Node *slow_ptr = head, *fast_ptr = head; 
    struct Node *second_half, *prev_of_slow_ptr = head; 
      
    // To handle odd size list 
    struct Node* midnode = NULL; 
      
    // initialize result 
    bool res = true
  
    if (head != NULL && head->next != NULL)
    {
          
        // Get the middle of the list. Move slow_ptr by 1 
        // and fast_ptr by 2, slow_ptr will have the middle 
        // node 
        while (fast_ptr != NULL && fast_ptr->next != NULL) 
        
            fast_ptr = fast_ptr->next->next; 
  
            // We need previous of the slow_ptr for 
            // linked lists with odd elements
            prev_of_slow_ptr = slow_ptr; 
            slow_ptr = slow_ptr->next; 
        
  
        // fast_ptr would become NULL when there
        // are even elements in 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 != NULL)
        
            midnode = slow_ptr; 
            slow_ptr = slow_ptr->next; 
        
  
        // Now reverse the second half and 
        // compare it with first half 
        second_half = slow_ptr; 
          
        // NULL terminate first half 
        prev_of_slow_ptr->next = NULL; 
          
        // Reverse the second half 
        reverse(&second_half); 
          
        // compare 
        res = compareLists(head, second_half); 
  
        // Construct the original list back 
        reverse(&second_half); // Reverse the second half again 
  
        // If there was a mid node (odd size case)
        // which was not part of either first half
        // or second half. 
        if (midnode != NULL)
        
            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 
void reverse(struct Node** head_ref) 
    struct Node* prev = NULL; 
    struct Node* current = *head_ref; 
    struct Node* next; 
      
    while (current != NULL) 
    
        next = current->next; 
        current->next = prev; 
        prev = current; 
        current = next; 
    
    *head_ref = prev; 
  
// Function to check if two input
// lists have same data
bool compareLists(struct Node* head1, 
                  struct Node* head2) 
    struct Node* temp1 = head1; 
    struct Node* temp2 = head2; 
  
    while (temp1 && temp2)
    
        if (temp1->data == temp2->data) 
        
            temp1 = temp1->next; 
            temp2 = temp2->next; 
        
        else
            return 0; 
    
  
    // Both are empty return 1
    if (temp1 == NULL && temp2 == NULL) 
        return 1; 
  
    // Will reach here when one is NULL 
    // and other is not 
    return 0; 
  
// Push a node to linked list. Note
// that this function changes the head 
void push(struct Node** head_ref, char new_data) 
      
    // Allocate node 
    struct Node* new_node = (struct Node*)malloc(
        sizeof(struct Node)); 
  
    // Put in the data 
    new_node->data = new_data; 
  
    // Link the old list off the new node 
    new_node->next = (*head_ref); 
  
    // Move the head to point to the new node 
    (*head_ref) = new_node; 
  
// A utility function to print a given linked list 
void printList(struct Node* ptr) 
    while (ptr != NULL) 
    
        cout << ptr->data << "->"
        ptr = ptr->next; 
    
    cout << "NULL" << "\n"
  
// Driver code 
int main() 
      
    // Start with the empty list 
    struct Node* head = NULL; 
    char str[] = "abacaba"
    int i; 
  
    for(i = 0; str[i] != '\0'; i++)
    
        push(&head, str[i]); 
        printList(head); 
        isPalindrome(head) ? cout << "Is Palindrome" 
                 << "\n\n" : cout << "Not Palindrome" 
                 << "\n\n"
    
    return 0; 
  
// This code is contributed by Shivani


C




/* Program to check if a linked list is palindrome */
#include <stdbool.h> 
#include <stdio.h> 
#include <stdlib.h> 
  
/* Link list node */
struct Node { 
    char data; 
    struct Node* next; 
}; 
  
void reverse(struct Node**); 
bool compareLists(struct Node*, struct Node*); 
  
/* Function to check if given linked list is 
palindrome or not */
bool isPalindrome(struct Node* head) 
    struct Node *slow_ptr = head, *fast_ptr = head; 
    struct Node *second_half, *prev_of_slow_ptr = head; 
    struct Node* midnode = NULL; // To handle odd size list 
    bool res = true; // initialize result 
  
    if (head != NULL && head->next != NULL) { 
        /* Get the middle of the list. Move slow_ptr by 1 
        and fast_ptr by 2, slow_ptr will have the middle 
        node */
        while (fast_ptr != NULL && fast_ptr->next != NULL) { 
            fast_ptr = fast_ptr->next->next; 
  
            /*We need previous of the slow_ptr for 
            linked lists with odd elements */
            prev_of_slow_ptr = slow_ptr; 
            slow_ptr = slow_ptr->next; 
        
  
        /* fast_ptr would become NULL when there are even elements in 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 != NULL) { 
            midnode = slow_ptr; 
            slow_ptr = slow_ptr->next; 
        
  
        // Now reverse the second half and compare it with first half 
        second_half = slow_ptr; 
        prev_of_slow_ptr->next = NULL; // NULL terminate first half 
        reverse(&second_half); // Reverse the second half 
        res = compareLists(head, second_half); // compare 
  
        /* Construct the original list back */
        reverse(&second_half); // Reverse the second half again 
  
        // If there was a mid node (odd size case) which 
        // was not part of either first half or second half. 
        if (midnode != NULL) { 
            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 */
void reverse(struct Node** head_ref) 
    struct Node* prev = NULL; 
    struct Node* current = *head_ref; 
    struct Node* next; 
    while (current != NULL) { 
        next = current->next; 
        current->next = prev; 
        prev = current; 
        current = next; 
    
    *head_ref = prev; 
  
/* Function to check if two input lists have same data*/
bool compareLists(struct Node* head1, struct Node* head2) 
    struct Node* temp1 = head1; 
    struct Node* temp2 = head2; 
  
    while (temp1 && temp2) { 
        if (temp1->data == temp2->data) { 
            temp1 = temp1->next; 
            temp2 = temp2->next; 
        
        else
            return 0; 
    
  
    /* Both are empty return 1*/
    if (temp1 == NULL && temp2 == NULL) 
        return 1; 
  
    /* Will reach here when one is NULL 
    and other is not */
    return 0; 
  
/* Push a node to linked list. Note that this function 
changes the head */
void push(struct Node** head_ref, char new_data) 
    /* allocate node */
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); 
  
    /* put in the data */
    new_node->data = new_data; 
  
    /* link the old list off the new node */
    new_node->next = (*head_ref); 
  
    /* move the head to pochar to the new node */
    (*head_ref) = new_node; 
  
// A utility function to print a given linked list 
void printList(struct Node* ptr) 
    while (ptr != NULL) { 
        printf("%c->", ptr->data); 
        ptr = ptr->next; 
    
    printf("NULL\n"); 
  
/* Driver program to test above function*/
int main() 
    /* Start with the empty list */
    struct Node* head = NULL; 
    char str[] = "abacaba"
    int i; 
  
    for (i = 0; str[i] != '\0'; i++) { 
        push(&head, str[i]); 
        printList(head); 
        isPalindrome(head) ? printf("Is Palindrome\n\n") : printf("Not Palindrome\n\n"); 
    
  
    return 0; 


Java




/* Java program to check if linked list is palindrome */
  
class LinkedList {
    Node head; // head of list
    Node slow_ptr, fast_ptr, second_half;
  
    /* Linked list Node*/
    class Node {
        char data;
        Node next;
  
        Node(char d)
        {
            data = d;
            next = null;
        }
    }
  
    /* Function to check if given linked list is
       palindrome or not */
    boolean isPalindrome(Node head)
    {
        slow_ptr = head;
        fast_ptr = head;
        Node prev_of_slow_ptr = head;
        Node midnode = null; // To handle odd size list
        boolean res = true; // initialize result
  
        if (head != null && head.next != null) {
            /* Get the middle of the list. Move slow_ptr by 1
               and fast_ptr by 2, slow_ptr will have the middle
               node */
            while (fast_ptr != null && fast_ptr.next != null) {
                fast_ptr = fast_ptr.next.next;
  
                /*We need previous of the slow_ptr for
                  linked lists  with odd elements */
                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 != null) {
                midnode = slow_ptr;
                slow_ptr = slow_ptr.next;
            }
  
            // Now reverse the second half and compare it with first half
            second_half = slow_ptr;
            prev_of_slow_ptr.next = null; // NULL terminate first half
            reverse(); // Reverse the second half
            res = compareLists(head, second_half); // compare
  
            /* Construct the original list back */
            reverse(); // Reverse the second half again
  
            if (midnode != null) {
                // 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 */
    void reverse()
    {
        Node prev = null;
        Node current = second_half;
        Node next;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        second_half = prev;
    }
  
    /* Function to check if two input lists have same data*/
    boolean compareLists(Node head1, Node head2)
    {
        Node temp1 = head1;
        Node temp2 = head2;
  
        while (temp1 != null && temp2 != null) {
            if (temp1.data == temp2.data) {
                temp1 = temp1.next;
                temp2 = temp2.next;
            }
            else
                return false;
        }
  
        /* Both are empty return 1*/
        if (temp1 == null && temp2 == null)
            return true;
  
        /* Will reach here when one is NULL
           and other is not */
        return false;
    }
  
    /* Push a node to linked list. Note that this function
       changes the head */
    public void push(char new_data)
    {
        /* Allocate the Node &
           Put in the data */
        Node new_node = new Node(new_data);
  
        /* link the old list off the new one */
        new_node.next = head;
  
        /* Move the head to point to new Node */
        head = new_node;
    }
  
    // A utility function to print a given linked list
    void printList(Node ptr)
    {
        while (ptr != null) {
            System.out.print(ptr.data + "->");
            ptr = ptr.next;
        }
        System.out.println("NULL");
    }
  
    /* Driver program to test the above functions */
    public static void main(String[] args)
    {
  
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
  
        char str[] = { 'a', 'b', 'a', 'c', 'a', 'b', 'a' };
        String string = new String(str);
        for (int i = 0; i < 7; i++) {
            llist.push(str[i]);
            llist.printList(llist.head);
            if (llist.isPalindrome(llist.head) != false) {
                System.out.println("Is Palindrome");
                System.out.println("");
            }
            else {
                System.out.println("Not Palindrome");
                System.out.println("");
            }
        }
    }
}


Python3




# Python3 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_ptr 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 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 off the new one
        new_node.next = self.head
          
        # Move the head to point to 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\n")
        else:
            print("Not Palindrome\n")
        print()
  
# This code is contributed by MuskanKalra1


C#




/* C# program to check if linked list is palindrome */
using System;
class LinkedList {
    Node head; // head of list
    Node slow_ptr, fast_ptr, second_half;
  
    /* Linked list Node*/
    public class Node {
        public char data;
        public Node next;
  
        public Node(char d)
        {
            data = d;
            next = null;
        }
    }
  
    /* Function to check if given linked list is
       palindrome or not */
    Boolean isPalindrome(Node head)
    {
        slow_ptr = head;
        fast_ptr = head;
        Node prev_of_slow_ptr = head;
        Node midnode = null; // To handle odd size list
        Boolean res = true; // initialize result
  
        if (head != null && head.next != null) {
            /* Get the middle of the list. Move slow_ptr by 1
               and fast_ptr by 2, slow_ptr will have the middle
               node */
            while (fast_ptr != null && fast_ptr.next != null) {
                fast_ptr = fast_ptr.next.next;
  
                /*We need previous of the slow_ptr for
                  linked lists  with odd elements */
                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 != null) {
                midnode = slow_ptr;
                slow_ptr = slow_ptr.next;
            }
  
            // Now reverse the second half and compare it with first half
            second_half = slow_ptr;
            prev_of_slow_ptr.next = null; // NULL terminate first half
            reverse(); // Reverse the second half
            res = compareLists(head, second_half); // compare
  
            /* Construct the original list back */
            reverse(); // Reverse the second half again
  
            if (midnode != null) {
                // 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 */
    void reverse()
    {
        Node prev = null;
        Node current = second_half;
        Node next;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        second_half = prev;
    }
  
    /* Function to check if two input lists have same data*/
    Boolean compareLists(Node head1, Node head2)
    {
        Node temp1 = head1;
        Node temp2 = head2;
  
        while (temp1 != null && temp2 != null) {
            if (temp1.data == temp2.data) {
                temp1 = temp1.next;
                temp2 = temp2.next;
            }
            else
                return false;
        }
  
        /* Both are empty return 1*/
        if (temp1 == null && temp2 == null)
            return true;
  
        /* Will reach here when one is NULL
           and other is not */
        return false;
    }
  
    /* Push a node to linked list. Note that this function
       changes the head */
    public void push(char new_data)
    {
        /* Allocate the Node &
           Put in the data */
        Node new_node = new Node(new_data);
  
        /* link the old list off the new one */
        new_node.next = head;
  
        /* Move the head to point to new Node */
        head = new_node;
    }
  
    // A utility function to print a given linked list
    void printList(Node ptr)
    {
        while (ptr != null) {
            Console.Write(ptr.data + "->");
            ptr = ptr.next;
        }
        Console.WriteLine("NULL");
    }
  
    /* Driver program to test the above functions */
    public static void Main(String[] args)
    {
  
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
  
        char[] str = { 'a', 'b', 'a', 'c', 'a', 'b', 'a' };
  
        for (int i = 0; i < 7; i++) {
            llist.push(str[i]);
            llist.printList(llist.head);
            if (llist.isPalindrome(llist.head) != false) {
                Console.WriteLine("Is Palindrome");
                Console.WriteLine("");
            }
            else {
                Console.WriteLine("Not Palindrome");
                Console.WriteLine("");
            }
        }
    }
}
// This code is contributed by Arnab Kundu


Javascript




<script>
/* javascript program to check if linked list is palindrome */
  
  
    var head; // head of list
    var slow_ptr, fast_ptr, second_half;
  
    /* Linked list Node */
     class Node {
            constructor(val) {
                this.data = val;
                this.next = null;
            }
        }
  
    /*
     * Function to check if given linked list is palindrome or not
     */
    function isPalindrome(head) {
        slow_ptr = head;
        fast_ptr = head;
var prev_of_slow_ptr = head;
var midnode = null; // To handle odd size list
        var res = true; // initialize result
  
        if (head != null && head.next != null) {
            /*
             * Get the middle of the list. Move slow_ptr by 1 and fast_ptr by 2, slow_ptr
             * will have the middle node
             */
            while (fast_ptr != null && fast_ptr.next != null) {
                fast_ptr = fast_ptr.next.next;
  
                /*
                 * We need previous of the slow_ptr for linked lists with odd elements
                 */
                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 != null) {
                midnode = slow_ptr;
                slow_ptr = slow_ptr.next;
            }
  
            // Now reverse the second half and compare it with first half
            second_half = slow_ptr;
            prev_of_slow_ptr.next = null; // NULL terminate first half
            reverse(); // Reverse the second half
            res = compareLists(head, second_half); // compare
  
            /* Construct the original list back */
            reverse(); // Reverse the second half again
  
            if (midnode != null) {
                // 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
     */
    function reverse() {
var prev = null;
var current = second_half;
var next;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        second_half = prev;
    }
  
    /* Function to check if two input lists have same data */
    function compareLists(head1,  head2) {
var temp1 = head1;
var temp2 = head2;
  
        while (temp1 != null && temp2 != null) {
            if (temp1.data == temp2.data) {
                temp1 = temp1.next;
                temp2 = temp2.next;
            } else
                return false;
        }
  
        /* Both are empty return 1 */
        if (temp1 == null && temp2 == null)
            return true;
  
        /*
         * Will reach here when one is NULL and other is not
         */
        return false;
    }
  
    /*
     * Push a node to linked list. Note that this function changes the head
     */
     function push( new_data) {
        /*
         * Allocate the Node & Put in the data
         */
var new_node = new Node(new_data);
  
        /* link the old list off the new one */
        new_node.next = head;
  
        /* Move the head to point to new Node */
        head = new_node;
    }
  
    // A utility function to print a given linked list
    function printList(ptr) {
        while (ptr != null) {
            document.write(ptr.data + "->");
            ptr = ptr.next;
        }
        document.write("NULL<br/>");
    }
  
    /* Driver program to test the above functions */
      
  
        /* Start with the empty list */
  
        var str = [ 'a', 'b', 'a', 'c', 'a', 'b', 'a' ];
        var string = str.toString();
        for (i = 0; i < 7; i++) {
            push(str[i]);
            printList(head);
            if (isPalindrome(head) != false) {
                document.write("Is Palindrome");
                document.write("<br/>");
            } else {
                document.write("Not Palindrome");
                document.write("<br/>");
            }
        }
  
  
// This code contributed by gauravrajput1
</script>


Output: 

a->NULL
Is Palindrome

b->a->NULL
Not Palindrome

a->b->a->NULL
Is Palindrome

c->a->b->a->NULL
Not Palindrome

a->c->a->b->a->NULL
Not Palindrome

b->a->c->a->b->a->NULL
Not Palindrome

a->b->a->c->a->b->a->NULL
Is Palindrome

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

 

METHOD 3 (Using Recursion) 
Use two pointers left and right. Move right and left using recursion and check for following in each recursive call. 
1) Sub-list is a palindrome. 
2) Value at current left and right are matching.

If both above conditions are true then return true.

The idea is to use function call stack as a container. Recursively traverse till the end of list. When we return from last NULL, we will be at the last node. The last node to be compared with first node of list.

In order to access first node of list, we need list head to be available in the last call of recursion. Hence, we pass head also to the recursive function. If they both match we need to compare (2, n-2) nodes. Again when recursion falls back to (n-2)nd node, we need reference to 2nd node from the head. We advance the head pointer in the previous call, to refer to the next node in the list.
However, the trick is identifying a double-pointer. Passing a single pointer is as good as pass-by-value, and we will pass the same pointer again and again. We need to pass the address of the head pointer for reflecting the changes in parent recursive calls.
Thanks to Sharad Chandra for suggesting this approach.  

C++




// Recursive program to check if a given linked list is palindrome 
#include <bits/stdc++.h> 
using namespace std; 
  
/* Link list node */
struct node { 
    char data; 
    struct node* next; 
}; 
  
// Initial parameters to this function are &head and head 
bool isPalindromeUtil(struct node** left, struct node* right) 
    /* stop recursion when right becomes NULL */
    if (right == NULL) 
        return true
  
    /* If sub-list is not palindrome then no need to 
    check for current left and right, return false */
    bool isp = isPalindromeUtil(left, right->next); 
    if (isp == false
        return false
  
    /* Check values at current left and right */
    bool isp1 = (right->data == (*left)->data); 
  
    /* Move left to next node */
    *left = (*left)->next; 
  
    return isp1; 
  
// A wrapper over isPalindromeUtil() 
bool isPalindrome(struct node* head) 
    isPalindromeUtil(&head, head); 
  
/* Push a node to linked list. Note that this function 
changes the head */
void push(struct node** head_ref, char new_data) 
    /* allocate node */
    struct node* new_node = (struct node*)malloc(sizeof(struct node)); 
  
    /* put in the data */
    new_node->data = new_data; 
  
    /* link the old list off the new node */
    new_node->next = (*head_ref); 
  
    /* move the head to point to the new node */
    (*head_ref) = new_node; 
  
// A utility function to print a given linked list 
void printList(struct node* ptr) 
    while (ptr != NULL) { 
        cout << ptr->data << "->"
        ptr = ptr->next; 
    
    cout << "NULL\n"
  
/* Driver program to test above function*/
int main() 
    /* Start with the empty list */
    struct node* head = NULL; 
    char str[] = "abacaba"
    int i; 
  
    for (i = 0; str[i] != '\0'; i++) { 
        push(&head, str[i]); 
        printList(head); 
        isPalindrome(head) ? cout << "Is Palindrome\n\n" : cout << "Not Palindrome\n\n"
    
  
    return 0; 
//this code is contributed by shivanisinghss2110 


C




// Recursive program to check if a given linked list is palindrome 
#include <stdbool.h> 
#include <stdio.h> 
#include <stdlib.h> 
  
/* Link list node */
struct node { 
    char data; 
    struct node* next; 
}; 
  
// Initial parameters to this function are &head and head 
bool isPalindromeUtil(struct node** left, struct node* right) 
    /* stop recursion when right becomes NULL */
    if (right == NULL) 
        return true
  
    /* If sub-list is not palindrome then no need to 
    check for current left and right, return false */
    bool isp = isPalindromeUtil(left, right->next); 
    if (isp == false
        return false
  
    /* Check values at current left and right */
    bool isp1 = (right->data == (*left)->data); 
  
    /* Move left to next node */
    *left = (*left)->next; 
  
    return isp1; 
  
// A wrapper over isPalindromeUtil() 
bool isPalindrome(struct node* head) 
    isPalindromeUtil(&head, head); 
  
/* Push a node to linked list. Note that this function 
changes the head */
void push(struct node** head_ref, char new_data) 
    /* allocate node */
    struct node* new_node = (struct node*)malloc(sizeof(struct node)); 
  
    /* put in the data */
    new_node->data = new_data; 
  
    /* link the old list off the new node */
    new_node->next = (*head_ref); 
  
    /* move the head to pochar to the new node */
    (*head_ref) = new_node; 
  
// A utility function to print a given linked list 
void printList(struct node* ptr) 
    while (ptr != NULL) { 
        printf("%c->", ptr->data); 
        ptr = ptr->next; 
    
    printf("NULL\n"); 
  
/* Driver program to test above function*/
int main() 
    /* Start with the empty list */
    struct node* head = NULL; 
    char str[] = "abacaba"
    int i; 
  
    for (i = 0; str[i] != '\0'; i++) { 
        push(&head, str[i]); 
        printList(head); 
        isPalindrome(head) ? printf("Is Palindrome\n\n") : printf("Not Palindrome\n\n"); 
    
  
    return 0; 


Java




// Java program for the above approach
public class LinkedList{
      
// Head of the list
Node head; 
Node left;
  
public class Node
{
    public char data;
    public Node next;
  
    // Linked list node
    public Node(char d)
    {
        data = d;
        next = null;
    }
}
  
// Initial parameters to this function are
// &head and head
boolean isPalindromeUtil(Node right)
{
    left = head;
  
    // Stop recursion when right becomes null
    if (right == null)
        return true;
  
    // If sub-list is not palindrome then no need to
    // check for the current left and right, return
    // false
    boolean isp = isPalindromeUtil(right.next);
    if (isp == false)
        return false;
  
    // Check values at current left and right
    boolean isp1 = (right.data == left.data);
  
    left = left.next;
  
    // Move left to next node;
    return isp1;
}
  
// A wrapper over isPalindrome(Node head)
boolean isPalindrome(Node head)
{
    boolean result = isPalindromeUtil(head);
    return result;
}
  
// Push a node to linked list. Note that
// this function changes the head
public void push(char new_data)
{
      
    // Allocate the node and put in the data
    Node new_node = new Node(new_data);
  
    // Link the old list off the the new one
    new_node.next = head;
  
    // Move the head to point to new node
    head = new_node;
}
  
// A utility function to print a 
// given linked list
void printList(Node ptr)
{
    while (ptr != null
    {
        System.out.print(ptr.data + "->");
        ptr = ptr.next;
    }
    System.out.println("Null");
}
  
// Driver Code
public static void main(String[] args)
{
    LinkedList llist = new LinkedList();
    char[] str = { 'a', 'b', 'a', 'c', 'a', 'b', 'a' };
    for(int i = 0; i < 7; i++)
    {
        llist.push(str[i]);
        llist.printList(llist.head);
          
        if (llist.isPalindrome(llist.head)) 
        {
            System.out.println("Is Palindrome");
            System.out.println("");
        }
        else 
        {
            System.out.println("Not Palindrome");
            System.out.println("");
        }
    }
}
}
  
// This code is contributed by abhinavjain194


Python3




# Python program for the above approach
  
# Head of the list
head = None
left = None
  
class Node:
    def __init__(self, val):
        self.data = val
        self.next = None
  
# Initial parameters to this function are
# &head and head
def isPalindromeUtil(right):
    global head, left
  
    left = head
  
    # Stop recursion when right becomes null
    if (right == None):
        return True
  
    # If sub-list is not palindrome then no need to
    # check for the current left and right, return
    # false
    isp = isPalindromeUtil(right.next)
    if (isp == False):
        return False
  
    # Check values at current left and right
    isp1 = (right.data == left.data)
  
    left = left.next
  
    # Move left to next node;
    return isp1
  
# A wrapper over isPalindrome(Node head)
def isPalindrome(head):
    result = isPalindromeUtil(head)
    return result
  
# Push a node to linked list. Note that
# this function changes the head
def push(new_data):
    global head
  
    # Allocate the node and put in the data
    new_node = Node(new_data)
  
    # Link the old list off the the new one
    new_node.next = head
  
    # Move the head to point to new node
    head = new_node
  
# A utility function to print a
# given linked list
def printList(ptr):
    while (ptr != None):
        print(ptr.data, end="->")
        ptr = ptr.next
  
    print("Null ")
  
  
# Driver Code
str = ['a', 'b', 'a', 'c', 'a', 'b', 'a']
  
for i in range(0, 7):
    push(str[i])
    printList(head)
  
    if (isPalindrome(head) and i != 0):
        print("Is Palindrome\n")
    else:
        print("Not Palindrome\n")
  
# This code is contributed by saurabh_jaiswal.


C#




/* C# program to check if linked list 
is palindrome recursively */
using System;
      
public class LinkedList 
    Node head; // head of list 
    Node left; 
  
    /* Linked list Node*/
    public class Node 
    
        public char data; 
        public Node next; 
  
        public Node(char d) 
        
            data = d; 
            next = null
        
    
  
    // Initial parameters to this function are &head and head 
    Boolean isPalindromeUtil(Node right) 
    
        left = head; 
  
        /* stop recursion when right becomes NULL */
        if (right == null
            return true
  
        /* If sub-list is not palindrome then no need to 
        check for current left and right, return false */
        Boolean isp = isPalindromeUtil(right.next); 
        if (isp == false
            return false
  
        /* Check values at current left and right */
        Boolean isp1 = (right.data == (left).data); 
  
        /* Move left to next node */
        left = left.next; 
  
        return isp1; 
    
  
    // A wrapper over isPalindromeUtil() 
    Boolean isPalindrome(Node head) 
    
        Boolean result = isPalindromeUtil(head); 
        return result; 
    
  
    /* Push a node to linked list. Note that this function 
    changes the head */
    public void push(char new_data) 
    
        /* Allocate the Node & 
        Put in the data */
        Node new_node = new Node(new_data); 
  
        /* link the old list off the new one */
        new_node.next = head; 
  
        /* Move the head to point to new Node */
        head = new_node; 
    
  
    // A utility function to print a given linked list 
    void printList(Node ptr) 
    
        while (ptr != null
        
            Console.Write(ptr.data + "->"); 
            ptr = ptr.next; 
        
        Console.WriteLine("NULL"); 
    
  
    /* Driver code */
    public static void Main(String[] args) 
    
        /* Start with the empty list */
        LinkedList llist = new LinkedList(); 
  
        char []str = { 'a', 'b', 'a', 'c', 'a', 'b', 'a' }; 
        //String string = new String(str); 
        for (int i = 0; i < 7; i++) { 
            llist.push(str[i]); 
            llist.printList(llist.head); 
            if (llist.isPalindrome(llist.head) != false
            
                Console.WriteLine("Is Palindrome"); 
                Console.WriteLine(""); 
            
            else 
            
                Console.WriteLine("Not Palindrome"); 
                Console.WriteLine(""); 
            
        
    
  
// This code is contributed by Rajput-Ji


Javascript




<script>
// javascript program for the above approach
  
  
    // Head of the list
    var head;
    var left;
  
     class Node {
            constructor(val) {
                this.data = val;
                this.next = null;
            }
        }
  
    // Initial parameters to this function are
    // &head and head
    function isPalindromeUtil( right) {
        left = head;
  
        // Stop recursion when right becomes null
        if (right == null)
            return true;
  
        // If sub-list is not palindrome then no need to
        // check for the current left and right, return
        // false
        var isp = isPalindromeUtil(right.next);
        if (isp == false)
            return false;
  
        // Check values at current left and right
        var isp1 = (right.data == left.data);
  
        left = left.next;
  
        // Move left to next node;
        return isp1;
    }
  
    // A wrapper over isPalindrome(Node head)
    function isPalindrome( head) {
        var result = isPalindromeUtil(head);
        return result;
    }
  
    // Push a node to linked list. Note that
    // this function changes the head
    function push( new_data) {
  
        // Allocate the node and put in the data
        var new_node = new Node(new_data);
  
        // Link the old list off the the new one
        new_node.next = head;
  
        // Move the head to point to new node
        head = new_node;
    }
  
    // A utility function to print a
    // given linked list
    function printList( ptr) {
        while (ptr != null) {
            document.write(ptr.data + "->");
            ptr = ptr.next;
        }
        document.write("Null ");
        document.write("<br>");
  
    }
  
    // Driver Code
      
        var str = [ 'a', 'b', 'a', 'c', 'a', 'b', 'a' ];
        for (var i = 0; i < 7; i++) {
            push(str[i]);
            printList(head);
  
            if (isPalindrome(head)) {
                document.write("Is Palindrome");
                document.write("<br/>");
                document.write("<br>");
            } else {
                document.write("Not Palindrome");
                document.write("<br/>");
                document.write("<br/>");
            }
        }
          
// This code contributed by aashish1995
  
</script>


Output: 

a->NULL
Not Palindrome

b->a->NULL
Not Palindrome

a->b->a->NULL
Is Palindrome

c->a->b->a->NULL
Not Palindrome

a->c->a->b->a->NULL
Not Palindrome

b->a->c->a->b->a->NULL
Not Palindrome

a->b->a->c->a->b->a->NULL
Is Palindrome

Time Complexity: O(n) 
Auxiliary Space: O(n) if Function Call Stack size is considered, otherwise O(1).
 


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