Leetcode: 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.

Solution:

public class Solution {
public ListNode reverseBetween(ListNode head, int m, int n) {
if (head == null || head.next == null || m == n) {
return head;
}

ListNode dummy = new ListNode(0);
dummy.next = head;
head = dummy;

ListNode pre = new ListNode(0);
for (int i = 0; i < m; i++) {
pre = head;
head = head.next;
}

ListNode p1 = pre.next;
ListNode p2 = p1.next;
for (int j = m; j < n; j++) {
ListNode p3 = p2.next;
p2.next = p1;
p1 = p2;
p2 = p3;
}

pre.next.next = p2;
pre.next = p1;
return dummy.next;
}
}