Reverse sec half of list, compare first half;

public boolean isPalindrome(ListNode head) {

    public boolean isPalindrome(ListNode head) {
       if(head==null||head.next==null) return true;
       ListNode fast = head.next, slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }

    slow = reverse(slow);

    while (slow != null&&head!=null) {
        if (slow.val != head.val) {
            return false;
        }
        head = head.next;
        slow = slow.next;
    }
    return true;
    }
   public ListNode reverse(ListNode head) {
        if(head==null){
            return null;
        }
        ListNode p1 = head;
        ListNode p2 = p1.next;
        head.next = null;
        while(p1!=null&&p2!=null){
            ListNode tmp = p2.next;
            p2.next = p1;
            p1 = p2;
            p2 = tmp;
        }return p1;
    }