Remove Duplicates from Sorted List II

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode h = new ListNode(0);
        h.next = head;
        ListNode pre = h;
        ListNode p = head;
        int flag = 0;
        while(p!=null) {
            if(p.next!=null&&p.val==p.next.val) {
                flag = 1;
                p.next = p.next.next;
            }
            else if(flag==1) {
                pre.next = p.next;
                p = p.next;
                flag = 0;
            }
            else {
                pre = pre.next;
                p = p.next;
            }
        }
        return h.next;
    }
}