[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.

用一个值val来标记当前要删除的元素，如果结点的值等于val，就删除掉。如果当前值与下一个结点的值相等，就将val设成当前值。

 1 /**

 2  * Definition of ListNode

 3  * class ListNode {

 4  * public:

 5  *     int val;

 6  *     ListNode *next;

 7  *     ListNode(int val) {

 8  *         this->val = val;

 9  *         this->next = NULL;

10  *     }

11  * }

12  */

13 class Solution{

14 public:

15     /**

16      * @param head: The first node of linked list.

17      * @return: head node

18      */

19     ListNode * deleteDuplicates(ListNode *head) {

20         // write your code here

21         if (head == NULL) return NULL;

22         ListNode dummy(0);

23         ListNode *cur = head, *pre = &dummy, *tmp;

24         int val = head->val - 1;

25         while (cur != NULL) {

26             if (cur->val == val || (cur->next != NULL && cur->val == cur->next->val)) {

27                 val = cur->val;

28                 tmp = cur->next;

29                 delete cur;

30                 cur = tmp;

31             } else {

32                 pre->next = cur;

33                 cur = cur->next;

34                 pre = pre->next;

35             }

36         }

37         pre->next = NULL;

38         return dummy.next;

39     }

40 };