LeetCode -- Remove Duplicates from Sorted List

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *deleteDuplicates(ListNode *head) {
12         set<int> vis;
13         ListNode *p = head, *pre;
14         while (p != NULL) {
15             if (vis.find(p->val) == vis.end()) {
16                 vis.insert(p->val);
17                 pre = p;
18             } else {
19                 pre->next = p->next;
20             }
21             p = p->next;
22         }
23         
24         return head;
25     }
26 };