反转链表

https://leetcode.cn/problems/reverse-linked-list

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

示例 1：

输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

示例 2：

输入：head = [1,2]
输出：[2,1]

示例 3：

输入：head = []
输出：[]

提示：

• 链表中节点的数目范围是 [0, 5000]
• -5000 <= Node.val <= 5000

</> 代码

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

// 迭代
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode newHead = null;
        while (head != null) {
            ListNode next = head.next;
            ListNode tmp = newHead;
            newHead = head;
            newHead.next = tmp;
            head = next;
        }
        return newHead;
    }
}

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