leetcode-82-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* (ListNode* head) {
        if(!head || !head->next) return head;
        ListNode * res;
        ListNode first(0);
        res = &first;
        res->next = head;
        ListNode *pre = res;
        ListNode *cur = head;
        
        bool dup = false;
        while(cur != NULL)
        {
            while(cur->next != NULL && cur->next->val == cur->val)
            {
                cur = cur->next;
                dup = true;
            }
            if(dup && cur->next == NULL) //针对{1,1,2,2}{1,1}的情况
            {
                pre->next = cur->next;
                pre = pre->next;
            }
            if(!dup)
            {
                pre->next = cur;
                pre = pre->next;
            }
            dup = false;
            cur = cur->next;
        }
        return res->next;
    }
};