Remove Duplicates from Sorted List II

    public ListNode deleteDuplicates(ListNode head) {        // Start typing your Java solution below        // DO NOT write main() function        if(head == null) return head;        ListNode dummy = new ListNode(0);        dummy.next = head;        ListNode previous = dummy;        ListNode current = head;        while(current != null) {            int tmp = current.val;            if(current.next != null && current.next.val == tmp) {                while(current != null && current.val == tmp) {                    current = current.next;                    previous.next = current;                }            }else {                previous = current;                current = current.next;            }        }        return dummy.next;    }