简单题，就是从单链表中删除重复元素。
附上代码：
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 };