Javascript Program For Finding The Length Of Longest Palindrome List In A Linked List Using O(1) Extra Space

// Javascript program to find longest palindrome 
// sublist in a list in O(1) time. 

// structure of the linked list
class Node {
    constructor() {
        this.data = 0;
        this.next = null;
    }
}


// function for counting the common elements
function countCommon(a, b) {
    let count = 0;

    // loop to count common in the list starting
    // from node a and b
    for (; a != null && b != null; a = a.next, b = b.next)

        // increment the count for same values
        if (a.data == b.data)
            ++count;
        else
            break;

    return count;
}

// Returns length of the longest palindrome
// sublist in given list
function maxPalindrome(head) {
    let result = 0;
    let prev = null, curr = head;

    // loop till the end of the linked list
    while (curr != null) {
        // The sublist from head to current
        // reversed.
        let next = curr.next;
        curr.next = prev;

        // check for odd length
        // palindrome by finding
        // longest common list elements
        // beginning from prev and
        // from next (We exclude curr)
        result = Math.max(result, 2 *
            countCommon(prev, next) + 1);

        // check for even length palindrome
        // by finding longest common list elements
        // beginning from curr and from next
        result = Math.max(result, 2 *
            countCommon(curr, next));

        // update prev and curr for next iteration
        prev = curr;
        curr = next;
    }
    return result;
}

// Utility function to create a new list node
function newNode(key) {
    let temp = new Node();
    temp.data = key;
    temp.next = null;
    return temp;
}

/* Driver code */

/*
 Let us create a linked lists to 
 test the functions Created list is a:
  2->4->3->4->2->15
 */
let head = newNode(2);
head.next = newNode(4);
head.next.next = newNode(3);
head.next.next.next = newNode(4);
head.next.next.next.next = newNode(2);
head.next.next.next.next.next = newNode(15);

console.log(maxPalindrome(head));

// This code contributed by aashish1995