Java program to check whether linked list is palindrome or not

Java program to check whether linked list is palindrome or not

Java program to check whether the linked list is palindrome or not. In this program, we have to check whether the given linked list is palindromic or not. A palindrome is a list which has the property of reversing itself. To check whether a number is palindrome or not, we traverse the list and check if any element from the starting half doesn’t  match with the ending half of a list then we say it’s not a palindrome number otherwise it is palindrome.

Java program to check whether the given linked list is palindrome or not

Implementation for Checking a list is palindrome or not :-

  • First get the middle node of the given Linked List let take Consideration for both the odd and even cases.
  • Then, we will reverse the second half of the Linked List.
  • We will compare the second half with the first half if both the halves are identical then the linked list is a palindrome.
  • Reconstruct the actual given Linked List by again reversing the second half and attaching it back to the first half.

Example :

Given Linked list is 1–>2–>3–>2–>1

After reversing it will stay as it is

1–>2–>3–>2–>1   (true) (It is a palindrome list)

Linked List is a Palindrome or not

Code in JAVA Programming Language 

Run
import java.util.*;

public class Main
{

  public static void main (String[]args) throws Exception
  {
    LinkedList ll = new LinkedList ();
      ll.addFirst (10);
      ll.addFirst (20);
      ll.addFirst (30);
      ll.addFirst (40);
      ll.addFirst (40);
      ll.addFirst (30);
      ll.addFirst (20);
      ll.addFirst (10);

      ll.display ();

      System.out.println (ll.isPalindrome ());

  }

}

class LinkedList
{
  private class Node
  {
    int data;
    Node next;

// Node constructor
// There are two fields in the node- data and address of next node
    public Node (int data, Node next)
    {
      this.data = data;
      this.next = next;
    }
  }

  private Node head;
  private Node tail;
  private int size;

// Linked list constructor
  public LinkedList ()
  {
    this.head = null;
    this.tail = null;
    this.size = 0;

  }

// Function to find the size of linked list
  public int size ()
  {
    return this.size;
  }

// Function to check whether linked list is empty or not
  public boolean isEmpty ()
  {
    return this.size () == 0;
  }

// Function to traverse and print the linked list
  public void display ()
  {
    Node temp = head;
    while (temp != null)
      {
	System.out.print (temp.data + "  ");

	temp = temp.next;
      }
    System.out.println ("END");
  }

// Function to add a node in beginning of linked list
  public void addFirst (int item)
  {
// Create a temp node which points to head
    Node temp = new Node (item, head);

// If linked list is empty, temp is the head and tail node both
    if (this.size == 0)
      {
	this.head = this.tail = temp;
      }

// else set the head such that it now points to temp node
    else
      {
	this.head = temp;

      }

    this.size++;
  }

  public boolean isPalindrome ()
  {
    HeapMover start = new HeapMover ();

    start.node = this.head;

    return this.isPalindrome (start, this.head, 0);

  }

//Function to check whether linked list is palindrome or not
  private boolean isPalindrome (HeapMover start, Node end, int floor)
  {

//Base case is when we reach end of linked list
    if (end == null)
      {
	return true;
      }

//Recursive calls
    boolean rv = this.isPalindrome (start, end.next, floor + 1);

//If any recursive call results in false then return false
    if (rv == false)
      {
	return false;
      }

//Till floor is greater than 1/2 * size of linked list
    if (floor >= this.size () / 2)
      {

//If data of start node and end node is not same then it is not palindrome
	if (start.node.data != end.data)
	  {
	    return false;

	  }

//Change start node so that it now points to the next node
	else
	  {
	    start.node = start.node.next;
	    return true;
	  }

      }
    return rv;

  }

//Class to keep a node in the heap
  private class HeapMover
  {
    Node node;
  }

}
Output: 
10  20  30  40  40  30  20  10  END
true

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