deleteDuplicates

/**
 * 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) return nullptr;

        map<int, int> freq;

        ListNode* temp = head;
        while (temp) {
            freq[temp->val]++;
            temp = temp->next;
        }

        ListNode* dummy = new ListNode(0);  
        ListNode* newTail = dummy;

        for (auto& pair : freq) {
            if (pair.second == 1) {
                newTail->next = new ListNode(pair.first);
                newTail = newTail->next;
            }
        }

        return dummy->next;

    }
};