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 left int整型 
     * @param right int整型 
     * @return ListNode类
     */
    public ListNode reverseBetween (ListNode head, int left, int right){

        if(head==null || head.next==null || left>right || left==right) {
            return head;
        }
       
        ListNode resNode = new ListNode(-1);
        resNode.next = head;


        ListNode prev = resNode;
        int step = left -1;
        for(int i=0; i<left-1; i++) {
            prev = prev.next;
        }

        ListNode rightNode = prev;
        for(int i=0; i<right-left+1; i++) {
            rightNode  = rightNode.next;
        }

        // 取子链表
        ListNode leftNode = prev.next;
        ListNode rightNext = rightNode.next;

        // 断开子链表
        prev.next = null;
        rightNode.next = null;

        // 反转子链表
        reverseLinkList(leftNode);

        // 接回子链表
        prev.next = rightNode;
        leftNode.next = rightNext;
        
        return resNode.next;
    }

    public void reverseLinkList(ListNode head) {
        ListNode curr = head;
        ListNode prev = null,next = null;
        while(curr!=null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
    }
}