palindrome-linked-list.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {boolean}
 */
function isPalindrome(head) {
  let fast = head;
  let slow = head;

  while (fast !== null && fast.next !== null) {
    fast = fast.next.next;
    slow = slow.next;
  }

  if (fast) {
    slow = slow.next;
  }

  slow = reverse(slow);
  fast = head;

  while (slow !== null) {
    if (fast.val !== slow.val) {
      return false;
    }

    fast = fast.next;
    slow = slow.next;
  }
  return true;
}

/**
 *
 * @param {ListNode} head
 * @returns {ListNode}
 */
function reverse(head) {
  let prev = null;

  while (head !== null) {
    const next = head.next;
    head.next = prev;
    prev = head;
    head = next;
  }

  return prev;
}