Problem

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2

Output: false

Example 2:

Input: 1->2->2->1

Output: true

Follow up: Could you do it in O(n) time and O(1) space?

Pre analysis

Will put it into an array and check if its a palindrome. Will Take space Complexity of O(n);

Another solution

fast - slow variant

var isPalindrome = function(head) {
    let slow = fast = head;

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

    let prev = null;
    while (slow) {
        let next = slow.next;
        slow.next = prev;
        prev = slow;
        slow = next;

    }

    slow = prev;
    fast = head;
    while (slow) {
        if (slow.val !== fast.val) return false;
        slow = slow.next;
        fast = fast.next;
    }
    return true;
};