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;
}
}