92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

/*** 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) {ListNode* preHead=new ListNode(0);//添加一个头节点，后面省事多了preHead->next=head;ListNode* p=preHead,*tail=preHead;while(--m>0){p=p->next;}do{tail=tail->next;}while(n--);ListNode* q=p,*next=p->next;q->next=tail;p=next;while(p!=tail){  //头插法next=p->next;p->next=q->next;q->next=p;p=next;}return preHead->next;}
};