#include<iostream>

#include<cstdio>

using namespace std;

struct ListNode {

     int val;

     ListNode *next;

     ListNode(int x) : val(x), next(NULL) {}

 };

class Solution {

public:

    ListNode *deleteDuplicates(ListNode *head) {

        ListNode *pre = new ListNode();

        pre->next = head;

        ListNode *p = pre, *q;

        while (p->next&&p->next->next){

            if (p->next->val == p->next->next->val){

                q = p->next->next->next;

                while (q&&q->val == p->next->val)q = q->next;

                p->next->next = q;

            }

            else{

                p = p->next;

            }

        }

        return pre->next;

    }

};