LeetCode(M) Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,

Given 1->2->3->3->4->4->5, return 1->2->5.

Given 1->1->1->2->3, return 2->3.

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