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;    }}