题解 | #链表内指定区间反转#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public 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) {
        ListNode node = head;
        //倒序链表
        ListNode pre = null;
        //顺序链表
        ListNode next = null;
        //顺序链表的指针
        ListNode nextIndex = null;
        int i = 1;
        while (node != null) {
            head = node;
            ListNode newNode = node.next;
            if (i >= m && i <= n) {
                //在范围内使用倒序指针
                node.next = pre;
                pre = node;
                if (i == n ) {
                    //当指针到倒序最后的索引时，把顺序链表和倒序链表合并
                    if (next == null) {
                        next = pre;
                        nextIndex = next;
                    }else{
                        nextIndex.next = pre;
                    }             
                }
            } else {
                //范围外的顺序链表
                node.next = null;
                if (next == null) {
                    next = node;
                    nextIndex = next;
                } else {
                    //顺序链表的指针永远指向顺序链表的最后一位
                    while (nextIndex.next != null) {
                        nextIndex = nextIndex.next;
                    }
                    nextIndex.next = node;
                    nextIndex = nextIndex.next;
                }

            }

            node = newNode;
            i++;
        }
        return next;
    }
}