LeetCode Remove Duplicated from Sorted List

    public ListNode deleteDuplicates(ListNode head) {

        if(head==null || head.next==null)

            return head;

        ListNode pre=head;;

        ListNode p=head.next;

        while(p!=null)

        {

            if(p.val==pre.val)

            {

                pre.next=p.next;

                p=p.next;

            }

            else

            {

                pre=p;

                p=p.next;

            }

        }

        return head;

    }