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

.

Diffiiculty: Medium

/**
* 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(0);
ListNode beforeHead = pre;
pre.next = head;
ListNode currHead = head;
for(int i = 0; i < m - 1; i++){
currHead = currHead.next;
pre = pre.next;
}
ListNode currTail = currHead;
ListNode next = currTail.next;
for(int i = 0; i < n - m; i++){
currTail = currTail.next;
next = next.next;
}
ListNode curr = currHead;
ListNode curr_next = curr.next;
for(int i = 0; i < n - m; i++){
ListNode temp = curr_next.next;
curr_next.next = curr;
curr = curr_next;
curr_next = temp;
}
pre.next = currTail;
currHead.next = next;
return beforeHead.next;
}
}