题意：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

 双指针

public class Main {

    public static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
        }
    }

    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode current = head;
        while (current != null) {
            ListNode nextNode = current.next;
            current.next = prev;
            prev = current;
            current = nextNode;
        }
        return prev;
    }

    public static void main(String[] args) {
        // 创建链表 1->2->3->4->5->NULL
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);
        head.next.next.next.next.next = null;

        // 反转链表并打印结果
        ListNode reversedHead = new Main().reverseList(head);
        while (reversedHead != null) {
            System.out.print(reversedHead.val + "->");
            reversedHead = reversedHead.next;
        }
        System.out.println("null");
    }
}