翻转链表II-Leetcode92

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.

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode reverseBetween(ListNode head, int m, int n) {
if(head == null || m >= n)
return head;

ListNode pre = new ListNode(-1);
ListNode newHead = pre;
pre.next = head;
for(int i=0; i<m-1; ++i) {
pre = pre.next;
}
ListNode start = pre.next;
ListNode then = start.next;
for(int i=0; i<n-m; ++i) {
start.next = then.next;
then.next = pre.next;
pre.next = then;
then = start.next;
}
return newHead.next;
}
}