LeetCode 82. 删除排序链表中的重复元素 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        auto dummy=new ListNode(-1);
        auto tail=dummy;
        unordered_map<int,int> s;
       
        for(auto p=head;p;p=p->next)
            s[p->val]++;
        for(auto p=head;p;p=p->next)
            if(s[p->val]==1)    tail=tail->next=p;
        tail->next=nullptr;
        return dummy->next;
    }
};