Leetcode-Remove Duplicates from Sorted List II

 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         if (head==null || head.next==null) return head;
15         ListNode preHead = new ListNode(-1);
16         preHead.next = head;
17         ListNode cur = head;
18         ListNode pre = preHead;
19 
20         //NOTE:We need to be every carefull about the condition,
21         //because if the last node be deleted, the cur is null in the next step.
22         //however, if the node before the last node is deleted, the cur is the last node in the next step,
23         //we then need to stop also, otherwise cur.next.val is invalid.
24         while (cur!=null && cur.next!=null){
25             if (cur.val==cur.next.val){
26                 while (cur.val==cur.next.val){
27                     cur.next = cur.next.next;
28                     if (cur.next==null)
29                         break;
30                 }
31                 pre.next = cur.next;                                
32                 cur = pre.next;
33             } else {
34                 pre = cur;
35                 cur = cur.next;
36                 if (cur.next==null)
37                     break;
38             }
39         }
40 
41         return preHead.next;            
42     }
43 }