3217.cpp

#include <vector>
#include <unordered_set>
using namespace std;

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* modifiedList(vector<int>& nums, ListNode* head) {
        unordered_set<int> s;
        for (int i : nums) { s.insert(i); }
        
        ListNode* dummy = new ListNode(0, head);
        ListNode* curr = dummy;

        while (curr->next) {
            if (s.find(curr->next->val) != s.end())
                curr->next = curr->next->next;
            else
                curr = curr->next;
        }

        return dummy->next;
    }
};