【leetcode】Remove Duplicates from Sorted List II-很精简

class Solution {
public:
ListNode *deleteDuplicates(ListNode *head) {
if(head == NULL) return NULL;
ListNode res(-1);
ListNode* pre = &res;
pre->next = head;
bool flag = true;
while(head != NULL){
if(head->next == NULL || head->val != head->next->val){
if(!flag) pre->next = head->next;
else pre = pre->next;
flag = true;
}else
flag = false;
head = head->next;
}
return res.next;
}
};