Solution.cs

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public bool IsPalindrome(ListNode head) {
        if (head == null) {
            return true;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode p = slow.next;
        slow.next = null;
        ListNode dummy = new ListNode(0);
        while (p != null) {
            ListNode next = p.next;
            p.next = dummy.next;
            dummy.next = p;
            p = next;
        }
        p = dummy.next;
        while (p != null) {
            if (head.val != p.val) {
                return false;
            }
            head = head.next;
            p = p.next;
        }
        return true;
    }
}