题目思路：

代码

class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        auto  dummy = new ListNode(-1);
        dummy->next = head;
        auto p = dummy;
        while(p->next){
            auto q = p->next->next;
            while(q && p->next->val == q->val) q = q->next;
            if(p->next->next == q) p = p->next;
            else p->next = q;
        }

        return dummy->next;
    }
};