反转一个单链表

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

import List.ListNode;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) { // 206
        if(head == null || head.next == null) return head;
        ListNode l1 = head;
        ListNode l2 = reverseList(l1.next);
        ListNode temp = l2;
        while(temp.next != null) {
            temp = temp.next;
        }
        temp.next = l1;
        l1.next = null;
        return l2;
    }
    
    // others
    public ListNode reverseList1(ListNode head) {
        if (head==null) return head;
        if (head.next==null) return head;
        ListNode p=reverseList(head.next);
        head.next.next=head;
        head.next=null;
        return p;
    }
}

对比大佬的代码，我开始代码的循环多余，可改成这样

class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode l2 = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return l2;
    }
}