/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplication(ListNode* head) {
        if (!head || !head->next) return head;
        auto dump = new ListNode(-1);
        dump->next = head;
        ListNode* p = dump;
        ListNode* q = p->next;


        while(q) {
            while (q->next && q->val == q->next->val) {
                q = q->next;
            }

            if (p->next == q) {
                p = p->next;
                q = q->next;
            }
            else {
                p->next = q->next;
                q = p->next;
            }
        }

        return dump->next;
    }
};