【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(-1);
        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;
    }
};