/**
 * 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) {
      if(head==NULL)return head;
       auto s=new ListNode(-1);
         s->next = head;
         auto p=s;
        while(p->next)
        {
          auto t=p->next,u=p->next;
          while(t&&t->val==p->next->val)t=t->next;
          if(u->next==t)p=p->next;
          else p->next=t;
        }
        return s->next;
    }
};