题解 | #牛群的重新排列#

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) {
        // write code here
        if(left == right || head == null){
            return head;
        }
        ListNode dummyNode = new ListNode(-700);
        dummyNode.next = head;
        ListNode fast = dummyNode;
        ListNode slow = dummyNode;
        for(int i = 0; i < left - 1; i++){
            slow = slow.next;
        }
        for(int i = 0; i < right; i++){
            fast = fast.next;
        }
        ListNode next = fast.next;
        fast.next = null;
        ListNode start = slow.next;
        slow.next = reverse(start);
        start.next = next;
        return dummyNode.next;
    }

    public ListNode reverse(ListNode head){
        if(head == null || head.next == null){
            return head;
        }
        ListNode node = reverse(head.next);
        head.next.next = head;
        head.next = null;
        return node;
    }
}

简单题，重拳出击