remove duplicates from sorted list

题目：

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,

Given1->1->2, return1->2.

Given1->1->2->3->3, return1->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 == NULL)
return NULL;
for (ListNode* pre = head, *cur = head->next ; cur; cur =cur->next )
{
if (pre->val == cur->val)
{
pre->next = cur->next;
delete cur;
}
else
{
pre = cur;
}

}

return head;
}
};