Remove Duplicates from Sorted List II

/**
* 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)
{

if(head==NULL || head->next==NULL)
return head;
ListNode* now=head;
ListNode* newHead=new ListNode(-1);
ListNode* pre=newHead;
newHead->next=head;
while(now && now->next)
{
if(now->val == now->next->val)
{
while(now->next && now->val == now->next->val)
{
now=now->next;
}
pre->next=now->next;
now=now->next;
}
else
{
pre=now;
now=now->next;
}
}
head=newHead->next;
delete(newHead);
return head;

}
};