/**
 * 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) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        auto p = dummy;
        while (p->next) {
            auto q = p->next;
            while (q && p->next->val == q->val) q = q->next;
            if (p->next->next == q)  p = p->next;
            else    p->next = q;
        }
        return dummy->next;
    }
};