234. palindrome linked list

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class  {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) {
           return true;
       }
        Deque<Integer> stack = new ArrayDeque<>();
      ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
           stack.push(slow.val);
            slow = slow.next;
            fast = fast.next.next;
       }
        //skip mid node if the number of ListNode is odd;
       if (fast != null) {
            slow = slow.next;
        }
        
     ListNode rCur = slow;
        while (rCur != null) {
         if (rCur.val != stack.pop()) {
              return false;
          }
            rCur = rCur.next;
        }
        return true;
    }
}