[LeetCode] 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 == NULL || head->next == NULL) return head;
        
        for (ListNode **prev = &head; (*prev) != NULL;) {
            int count = 0;
            ListNode *pter = *prev;
            while (pter != NULL && (pter->val) == (*prev)->val) {
                pter = pter->next;
                ++count;
            }
                if (count == 1) {
                    prev = &((*prev)->next);
                } else {
                    while ((*prev) != pter) {
                        ListNode *entry = *prev;
                        *prev = entry->next;
                        delete entry;
                    }
            }
        }
        
        return head;
    }
};