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