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:

 1   ListNode* deleteDuplicates(ListNode* head) {

 2         if (head)

 3         {

 4             ListNode* p = head;

 5             ListNode* lastNode = head;

 6             

 7             int lastDeleteVal = 0;

 8             bool flag = false;//flag为false表示还没有元素被删除

 9             

10             while (p)

11             {

12                 if ( (flag == true && p->val == lastDeleteVal)

13                 || (p->next!=NULL && p->val == p->next->val))

14                 {//删除p

15                     lastDeleteVal = p->val;

16                     if (p == head)

17                         head = p->next;

18                     else

19                         lastNode->next = p->next;

20                     p = p->next;

21                     if (flag == false)

22                         flag = true;

23                 }

24                 else

25                 {

26                     lastNode = p;

27                     p = p->next;

28                 }

29             }

30         }

31         return head;

32     }