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# Function to check if a singly linked list is palindrome

• Difficulty Level : Medium
• Last Updated : 17 Jan, 2023

Given a singly linked list of characters, write a function that returns true if the given list is a palindrome, else false. Examples:

Input: R->A->D->A->R->NULL
Output: Yes

Input: C->O->D->E->NULL
Output: No

## Check if a Singly Linked List is Palindrome using Stack:

The idea is to use a stack and push all the nodes into the stack, then again iterate over the linked list to validate if the linked list is palindrome or not. Below image is a dry run of the above approach: Follow the steps below to solve the problem:

• 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.

Below is the implementation of the above approach :

## C++

 `#include ``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 false\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 stack = ``new` `Stack();` `        ``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

 ``

Output

`isPalindrome is true`

Time complexity: O(N), Iterating over the linked list of size N.
Auxiliary Space: O(N), Using an auxiliary stack

## Check if a Singly Linked List is Palindrome by Reversing the Linked List:

The idea is to first reverse the second half part of the linked list and then check whether the list is palindrome or not.

Follow the steps below to solve the problem:

• Get the middle of the linked list.
• Reverse the second half of the linked list.
• Check if the first half and second half are identical.
• Construct the original linked list by reversing the second half again and attaching it back to the first half

Below is the implementation of the above approach:

## C++

 `// C++ program to check if a linked list is palindrome``#include ``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 of 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]);``    ``}``    ``isPalindrome(head) ? cout << ``"Is Palindrome"``                              ``<< ``"\n\n"``                       ``: cout << ``"Not Palindrome"``                              ``<< ``"\n\n"``;``    ``return` `0;``}` `// This code is contributed by Shivani`

## C

 `/*  C Program to check if a linked list is palindrome */``#include ``#include ``#include ` `/* 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 of 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]);``    ``}``    ``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 of 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]);``        ``}``        ``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 of 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])``    ``if` `(l.isPalindrome(l.head) !``=` `False``):``        ``print``(``"Is Palindrome\n"``)``    ``else``:``        ``print``(``"Not Palindrome\n"``)` `# 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 of 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]);``        ``}``        ``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

 ``

Output

`Is Palindrome`

Time Complexity: O(N),
Auxiliary Space: O(1)

## Check if a Singly Linked List is Palindrome using Recursion:

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

Follow the steps below to solve the problem:

• First make a recursive call to the next node of linked till it reach the last node.
• After returning from last node start checking from start of the linked list then move to the next node.
• Repeat these steps till reach the last node.

Below is the implementation of above approach:

## C++

 `// Recursive program to check if a given linked list is``// palindrome``#include ``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)``{``    ``return` `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 of 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]);``    ``}``    ``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 ``#include ``#include ` `/* 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)``{``    ``return` `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 of 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]);``    ``}``    ``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 of 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]);``        ``}``        ``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 of 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])` `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 of 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]);``        ``}` `        ``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

 ``

Output

`Is Palindrome`

Time Complexity: O(N), Traversing over the linked list of size N.
Auxiliary Space: O(N) if Function Call Stack size is considered, otherwise O(1).

Thanks to Sharad Chandra for suggesting this approach.

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