/**

 * 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 *pre, *cur, *ahead;

        ahead=pre=new ListNode(-);cur=head;

        while(cur!=NULL){

            while(cur->next!=NULL && cur->val==cur->next->val){

                cur=cur->next;

            }

            pre->next=cur;

            pre=cur;

            cur=cur->next;

        }

        return ahead->next;

    }

};