/**
 * 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) {
        ListNode* NIL = new ListNode(-1);
        NIL->next = head;
        auto* t = NIL;
        while (t->next && t->next->next) {
            if (t->next->val == t->next->next->val) {
                int v = t->next->val;
                while (t->next && t->next->val == v) t->next = t->next->next;
            } else {
                t = t->next;
            }
        }
        return NIL->next;
    }
};