一、题目内容

二、题解

解法1：

思路

注意换头情况
先记录left左边的pre，以及right

代码

import java.util.*;
public class Solution {
    public ListNode reverseBetween (ListNode head, int m, int n) {
        if(m == n){
            return head;
        }
        // write code here
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        int len = 0;
        ListNode cur = head;
        ListNode pre = null,right = null;
        while(cur!=null){
            len+=1;
            if(len == m-1){
                pre = cur;
            }
            if(len == n){
                right = cur;
            }
            cur = cur.next;
        }
        if(m > n || m < 1 || n > len){
            return head;
        }
        //换头
        if(pre == null){
            pre = dummy;
        }
        ListNode left = pre.next;
        pre.next = null;
        ListNode next = right.next;
        right.next = null;
        ListNode newLeft = reverse(left);
        left.next = next;
        pre.next = newLeft;
        return dummy.next;
    }
    private ListNode reverse(ListNode root){
        ListNode pre = null,cur = root;
        while(cur!=null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}