/**
 * 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;
        ListNode* p = dummy;

        while(p->next)
        {
            ListNode* q = p->next;
            while(q->next && q->next->val == p->next->val) q = q->next;

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

        return dummy->next;
    }
};