题解 | #链表内指定区间反转#

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) {
        if (head == null) {
            return head;
        }

// 将列表分成3段
        ListNode first = null;
        ListNode senond = null;
        ListNode third = null;

        int index = 1;
        ListNode node = head;
        while (node != null) {
            if (index < m) {
                if (first == null) {
                    first = new ListNode(node.val);
                } else {
                    this.GetLastNode(first).next = new ListNode(node.val);
                }
            } else if (index > n) {
                if (third == null) {
                    third = new ListNode(node.val);
                } else {
                    this.GetLastNode(third).next = new ListNode(node.val);
                }
            } else {
                if (senond == null) {
                    senond = new ListNode(node.val);
                } else this.GetLastNode(senond).next = new ListNode(node.val);
            }

            index++;
            node = node.next;
        }

// 中间列表翻转
        ListNode pre = null;
        ListNode cur = senond;
        ListNode next = null;

        while (cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        if (first != null)
        {
            this.GetLastNode(first).next = pre;
        }
        else
        {
            first = pre;
        }
        this.GetLastNode(first).next = third;

        return first;
    }

    public ListNode GetLastNode(ListNode node) {
        while (node.next != null) {
            node = node.next;
        }

        return node;
    }
}