using System;
using System.Collections.Generic;

/*
public class ListNode
{
	public int val;
	public ListNode next;

	public ListNode (int x)
	{
		val = x;
	}
}
*/

class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param m int整型 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        // write code here
        ListNode virtualHead = new ListNode(-1);
        virtualHead.next = head;
        ListNode cur = head;
        //m之前的节点
        ListNode pre_Left = virtualHead;
        //m之后的节点
        ListNode pre_Right = cur;

        for(int i = 1; i < m; i++)
        {   
            pre_Left = cur; 
            cur = cur.next;
            pre_Right = cur;
        }
        pre_Left.next = null;
        ListNode _pre,_cur;
        ReverseNode(cur, out _pre,out _cur, n- m +1 );
        pre_Left.next = _pre;   
        pre_Right.next = _cur;
        return virtualHead.next;

    }

    void ReverseNode(ListNode head,out ListNode _pre,out ListNode _cur,int cnt)
    {
        ListNode pre = null;
        ListNode next = null;
        for(int i = 1;i <= cnt ;i++)
        {       
            next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        _pre = pre;
        _cur = head;
    }
}

借助C#out将拆分区间的Index进行了返回，这样不需要提前遍历一遍获得Index，但还是不如直接边遍历边反转简单，还需要拼接两次头尾。