【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 == nullptr || head->next == nullptr)
             return head;
         ListNode dummy(-);
         dummy.next = head;
         ListNode *prev = &dummy, *cur = head;
         while (cur != nullptr) {
             bool flag = false;
             while (cur->next != nullptr && cur->val == cur->next->val) {
                 flag = true;
                 prev->next = cur->next;
                 delete cur;
                 cur = prev->next;
             }
             if (flag) {
                 prev->next = cur->next;
                 delete cur;
                 cur = prev->next;
             } else {
                 prev = cur;
                 cur = cur->next;
             }
         }
         return dummy.next;
     }
 };