/**
 * 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) {
        if(head == nullptr || head->next == nullptr) return head;
        map<int,int> visit;
        ListNode* cur = head;
        ListNode* dummy = new ListNode(-1);
        ListNode* res = dummy;
        while(cur){
            visit[cur->val]++;
            cur = cur->next;
        }
        cur = head;
        while(cur){
            if(visit[cur->val] == 1){
                res->next = new ListNode(cur->val);
                res = res->next;
            }
            cur =cur->next;
        }
        return dummy->next;
    }
};