Solution.ts

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function isPalindrome(head: ListNode | null): boolean {
    if (!head) {
        return true;
    }
    let slow = head;
    let fast = head.next;
    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }
    let p = slow.next;
    slow.next = null;
    const dummy = new ListNode(0);
    while (p) {
        const next = p.next;
        p.next = dummy.next;
        dummy.next = p;
        p = next;
    }
    p = dummy.next;
    while (p) {
        if (head.val !== p.val) {
            return false;
        }
        head = head.next;
        p = p.next;
    }
    return true;
}