Remove Duplicates from Sorted List II

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *deleteDuplicates(ListNode *head) {
12         if (head == NULL || head->next == NULL) return head;
13         
14         for (ListNode **prev = &head; (*prev) != NULL;) {
15             int count = 0;
16             ListNode *pter = *prev;
17             while (pter != NULL && (pter->val) == (*prev)->val) {
18                 pter = pter->next;
19                 ++count;
20             }
21                 if (count == 1) {
22                     prev = &((*prev)->next);
23                 } else {
24                     while ((*prev) != pter) {
25                         ListNode *entry = *prev;
26                         *prev = entry->next;
27                         delete entry;
28                     }
29             }
30         }
31         
32         return head;
33     }
34 };