/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplication(ListNode* head) {
        auto dummy= new ListNode(-1);
        dummy->next=head;
        auto p=dummy;
        while(p->next){
            auto q=p->next;
            while(q->next&&q->next->val==p->next->val) q=q->next;
            if(q==p->next) p=q;
            else p->next=q->next;
        }
        return dummy->next;
    }
};