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