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;
}