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

// Javascript program to check if a 
// linked list with loop is palindrome 
// or not.

// 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) {
    let ptr1 = loop_node;
    let ptr2 = loop_node;

    // Count the number of nodes in 
    // loop
    let 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) {
    let 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) {
    let ptr = head;
    let s = [];

    // Traverse linked list until last node
    // is equal to loop_start and store the
    // elements till start in a stack
    let 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
        let 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
    let loop_start =
        detectAndgetLoopstarting(head);

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

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

// Driver code 
let 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)
    console.log("Palindrome");
else
    console.log("Not Palindrome");
// This code is contributed by aashish1995