leetcode -- Remove Duplicates from Sorted List

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

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode deleteDuplicates(ListNode head) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         if(head == null){
17             return head;
18         }
19         Set<Integer> s = new HashSet<Integer>();
20         ListNode cur = head, pre = null;
21         while(cur != null){
22             if(s.contains(cur.val)){
23                 pre.next = cur.next;
24                 cur = cur.next;
25             } else {
26                 s.add(cur.val);
27                 pre = cur;
28                 cur = cur.next;
29             }
30         }
31         return head;
32     }
33 }

这里使用了额外的存储空间，可以使用双指针来实现，不需额外的存储空间