https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/
题目：
Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct 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.
思路：
需要两个指针，a用来纪录新链表，b用来遍历老链表，因为只有删除操作，需要一个flag标记ab段要不要删除。此外要注意头节点的处理。
AC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;
ListNode* newhead=new ListNode(); // to handle head
newhead->next=head;
ListNode* a=newhead,*b=newhead->next;
int now=head->val;
bool occur_flag=false;
while(b->next!=NULL){
if(b->next->val==now){
occur_flag=true;
}
else{
if(occur_flag==true){
a->next=b->next; //delete duplicate numbers
b=a; //update pointers
}
else{
a=b;
}
occur_flag=false;
now=b->next->val;
}
b=b->next;
}
if(occur_flag==true){
a->next=b->next; //delete duplicate numbers
b=a; //update pointers
}
else{
a=b;
}
return newhead->next;
}
};