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         ListNode first;
15             ListNode second;
16             if(null == head)
17                 return null;
18             first = head;
19             second = head.next;
20             while(second != null){
21                 while(null != second && first.val == second.val){
22                     first.next = second.next;
23                     second = first.next;
24                 }
25                 first = first.next;
26                 if(null != second)
27                     second = second.next;
28             }
29             return head;
30     }
31 }