/**

 * Definition for singly-linked list.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) { val = x; }

 * }

 */

class Solution {

    public ListNode deleteDuplicates(ListNode head) {

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

            return head;

        }

        ListNode dummy = new ListNode(-1);

        dummy.next = head;

        ListNode cur = dummy;

        // check the next two node

        while(cur.next != null && cur.next.next != null) {

            if (cur.next.val == cur.next.next.val) {

                int num = cur.next.val;

                while(cur.next != null && cur.next.val == num) {

                    cur.next = cur.next.next;

                }

            } else {

                cur = cur.next;

            }

        }

        return dummy.next;

    }

}