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. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */class Solution {    public ListNode reverseBetween(ListNode head, int m, int n) {        if (m >= n || head == null) {            return head;        }        ListNode dummy = new ListNode(0);        dummy.next = head;        head = dummy;        for (int i = 1; i < m; i++) {            if (head == null) {                return null;            }            head = head.next;        }        ListNode premNode = head;        ListNode mNode = head.next;        ListNode nNode = mNode, postnNode = mNode.next;        for (int i = m; i < n; i++) {            if (postnNode == null) {                return null;            }            ListNode temp = postnNode.next;            postnNode.next = nNode;            nNode = postnNode;            postnNode = temp;        }        mNode.next = postnNode;        premNode.next = nNode;        return dummy.next;    }}