82.Remove Duplicates from Sorted List II

 1     public ListNode deleteDuplicates(ListNode head) {
 2         //pre记录前面最后一个重复的结点，cur进行遍历
 3         ListNode res = null, cur = res, pre = null;
 4         while(head != null) {
 5             //如果前后节点重复则跳过，即删除
 6             if(head.next != null && head.val == head.next.val) {
 7                 pre = head;
 8                 head = head.next.next;
 9             }
10             //如果当前节点与上一个节点重复，则删除
11             else if(pre != null && head.val == pre.val) {
12                 head = head.next;
13             }
14             //如果不重复，则将结点保留
15             else {
16                 //尾插
17                 if(cur == null) {
18                     cur = head;
19                     res = cur;
20                 }
21                 else {
22                     cur.next = head;
23                     cur = head;
24                 }
25                 head = head.next;
26             }
27         }
28         if(cur == null) {
29             return null;
30         }
31         cur.next = null;
32         return res;
33     }