[leetcode] Reverse Linked List II

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *reverseBetween(ListNode *head, int m, int n) {
if(head==nullptr){
return nullptr;
}
ListNode *before,*start,*end,*after;
before=after=start=end=nullptr;//初始化
ListNode *ptr=head;

for(int i=1;i<m;++i){
before=ptr;
ptr=ptr->next;
}//ptr指向第m个元素,before指向第m-1个元素(从1开始计数)

start=end=ptr;
//ptr=ptr->next;

for(int i=m;i<=n;++i){
after=ptr->next;
ptr->next=end;
end=ptr;
ptr=after;
}
if(before==nullptr){//m=1时
head=end;
}else{
before->next=end;
}
start->next=after;
return head;
}
};