输入一个链表，反转链表后，输出链表的所有元素

public class ListNode {    int val;    ListNode next = null;    ListNode(int val) {        this.val = val;    }}public class Solution {    public ListNode ReverseList(ListNode head) {        ListNode pre = null;        ListNode next = null;        while(head != null){            next = head.next;            head.next = pre;            pre = head;            head = next;        }        return pre;    }}