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;}
};