Remove Duplicates from Sorted List II

Description:

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.

Code:

  ListNode* deleteDuplicates(ListNode* head) {
        if (head)
        {
            ListNode* p = head;
            ListNode* lastNode = head;
            
            int lastDeleteVal = 0;
            bool flag = false;//flag为false表示还没有元素被删除
            
            while (p)
            {
                if ( (flag == true && p->val == lastDeleteVal)
                || (p->next!=NULL && p->val == p->next->val))
                {//删除p
                    lastDeleteVal = p->val;
                    if (p == head)
                        head = p->next;
                    else
                        lastNode->next = p->next;
                    p = p->next;
                    if (flag == false)
                        flag = true;
                }
                else
                {
                    lastNode = p;
                    p = p->next;
                }
            }
        }
        return head;
    }