Javascript Program For Checking Linked List With A Loop Is Palindrome Or Not

Given a linked list with a loop, the task is to find whether it is palindrome or not. You are not allowed to remove the loop.  

Examples:

Input: 1 -> 2 -> 3 -> 2
             /|      |/
               ------- 1  
Output: Palindrome
Linked list is 1 2 3 2 1 which is a 
palindrome.

Input: 1 -> 2 -> 3 -> 4
             /|      |/
               ------- 1  
Output: Not Palindrome
Linked list is 1 2 3 4 1 which is a 
not palindrome. 

Algorithm:

1. Detect the loop using the Floyd Cycle Detection Algorithm.
2. Then find the starting node of the loop as discussed in this.
3. Check the linked list is palindrome or not as discussed in this.

Below is the implementation. 

Javascript

<script>
// Javascript program to check if a
// linked list with loop is palindrome
// or not.
class GfG{

// Link list node
class Node
{
    constructor()
    {
        this.data = 0;
        this.next = null;
    }
}

/* Function to find loop starting node.
   loop_node --> Pointer to one of the loop
   nodes head --> Pointer to the start node
   of the linked list */
function getLoopstart(loop_node, head)
{
    var ptr1 = loop_node;
    var ptr2 = loop_node;

    // Count the number of nodes in
    // loop
    var k = 1, i;
    while (ptr1.next != ptr2)
    {
        ptr1 = ptr1.next;
        k++;
    }

    // Fix one pointer to head
    ptr1 = head;

    // And the other pointer to k
    // nodes after head
    ptr2 = head;
    for (i = 0; i < k; i++)
        ptr2 = ptr2.next;

    /* Move both pointers at the same pace,
       they will meet at loop starting node */
    while (ptr2 != ptr1)
    {
        ptr1 = ptr1.next;
        ptr2 = ptr2.next;
    }
    return ptr1;
}

/* This function detects and find loop
   starting node in the list */
function detectAndgetLoopstarting(head)
{
    var slow_p = head, fast_p = head,
        loop_start = null;

    // Start traversing list and detect loop
    while (slow_p != null && fast_p != null &&
           fast_p.next != null)
    {
        slow_p = slow_p.next;
        fast_p = fast_p.next.next;

         /* If slow_p and fast_p meet then find
            the loop starting node */
         if (slow_p == fast_p)
         {
             loop_start = getLoopstart(slow_p, head);
             break;
         }
    }

    // Return starting node of loop
    return loop_start;
}

// Utility function to check if a linked
// list with loop is palindrome with given
// starting point.
function isPalindromeUtil(head, loop_start)
{
    var ptr = head;
    var s =  [];

    // Traverse linked list until last node
    // is equal to loop_start and store the
    // elements till start in a stack
    var count = 0;
    while (ptr != loop_start ||
           count != 1)
    {
        s.push(ptr.data);
        if (ptr == loop_start)
            count = 1;
         ptr = ptr.next;
    }
    ptr = head;
    count = 0;

    // Traverse linked list until last
    // node is equal to loop_start second
    // time
    while (ptr != loop_start ||
           count != 1)
    {
        // Compare data of node with
        // the top of stack
        // If equal then continue
        var stk = s.pop();
        if (ptr.data == stk);

        // Else return false
        else
        {
            s.push(stk);
            return false;
        }

        if (ptr == loop_start)
                count = 1;
            ptr = ptr.next;
    }
    // Return true if linked list is
    // palindrome
    return true;
}

// Function to find if linked list
// is palindrome or not
function isPalindrome(head)
{
    // Find the loop starting node
    var loop_start =
        detectAndgetLoopstarting(head);

    // Check if linked list is palindrome
    return isPalindromeUtil(head,
                            loop_start);
}

function newNode(key)
{
    var temp = new Node();
    temp.data = key;
    temp.next = null;
    return temp;
}

// Driver code
var head = newNode(50);
head.next = newNode(20);
head.next.next = newNode(15);
head.next.next.next = newNode(20);
head.next.next.next.next =
newNode(50);

// Create a loop for testing
head.next.next.next.next.next =
head.next.next;

if (isPalindrome(head) == true)
    document.write("Palindrome");
else
    document.write("Not Palindrome");
// This code is contributed by aashish1995
</script>

Output:

Palindrome

Time Complexity: O(n) where n is no of nodes in the linked list

Auxiliary Space: O(n) where n is size of List

Please refer complete article on Check linked list with a loop is palindrome or not for more details!