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 below15         // DO NOT write main() function16         if(head == null){17             return head;18         }19         Set
     
       s = new HashSet
      
       ();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