【Leetcode长征系列】Remove Duplicates from Sorted List II

原题：

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving onlydistinct numbers from the original list.

For example,

Given

1->2->3->3->4->4->5

, return

1->2->5

.

Given

1->1->1->2->3

, return

2->3

.

代码：

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *deleteDuplicates(ListNode *head) {
if(head==NULL) return NULL;
if(head->next==NULL) return head;
ListNode *tmp = head, *pre;

pre = NULL;
while(tmp){
if (tmp->val==tmp->next->val) {
ListNode *s = tmp->next;
while (s->next!=NULL && s->val==s->next->val){
s = s->next;
}
if(pre==NULL){
if(s->next!=NULL){
head = s->next;
tmp = head;
}
else return NULL;
}
else{
pre->next = s->next;
tmp = pre->next;
}
}
else {
pre = tmp;
tmp = tmp->next;
}

if (tmp==NULL || tmp->next==NULL) break;
}
return head;
}
};

需要注意的特殊情况：

1. {1}

2. {1->2}

3. {1->1->2}

要注意pre的初始值！NULL最佳，本次情况下不适合用head作为初始值。