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) {
        if(head == null || head.next == null || m == n) {
            return head;
        }

        int currentIndex = 1;
        ListNode leftTail = null;
        ListNode middleHead = head;
        while(currentIndex++ < m) {
            leftTail = middleHead;
            middleHead = middleHead.next;
        }

        ListNode reverserCurrent = middleHead;
        ListNode newNext = null;
        ListNode next = reverserCurrent.next;
        while(currentIndex++ <= n) {
            reverserCurrent.next = newNext;
            newNext = reverserCurrent;
            reverserCurrent = next;
            next = reverserCurrent.next;
        }

        // 最后一次循环结束后，一定要将reverserCurrent的指针只想newNext。
        reverserCurrent.next = newNext;

        if (leftTail != null) {
            leftTail.next = reverserCurrent;
        } else {
            head = reverserCurrent;
        }
        
        middleHead.next = next;

        return head;
    }
}