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

.

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public ListNode deleteDuplicates(ListNode head) {
// Start typing your Java solution below
// DO NOT write main() function
if(head == null){
return head;
}
Set<Integer> s = new HashSet<Integer>();
ListNode cur = head, pre = null;
while(cur != null){
if(s.contains(cur.val)){
pre.next = cur.next;
cur = cur.next;
} else {
s.add(cur.val);
pre = cur;
cur = cur.next;
}
}
return head;
}
}

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