/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* dummy = new ListNode(-1);
        dummy->next = head;
        if(!head)   return head;
        ListNode* pre = head, *r = head, *p = head->next;
        while(p){
            if(pre->val == p->val)  p = p->next;
            else{
                r->next = p;
                pre = p;
                r = p;
                p = p->next;
            }
        }
        r->next = NULL;
        return dummy->next;
    }
};