题解 | #链表内指定区间反转#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     *
     * @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
        int i = 1;
        ListNode cur = head;
        ListNode beforeNode = null;
        while (i < m) {
            if (i == m - 1) {
			    // 找到开始反转的前一个节点
                beforeNode = cur;
            }
            cur = cur.next;
            i++;
        }
        ListNode pre = null;
	    //从指定部分反转
        while (i <= n) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
            i++;
        }
        if (beforeNode != null) {
		  	//没有从头开始开始前一个节点拼接pre
            beforeNode.next = pre;
        } else {
			//从头开始反转，head 就是pre
            head = pre;
        }
        ListNode newNode = pre;
	    //从开始反转位置遍历，遇到下一个节点是空，直接拼接后缀部分
        while (newNode.next != null) {
            newNode = newNode.next;
        }
        newNode.next = cur;
        return head;
    }
}