public ListNode reverseList(ListNode head) {
    //判断链表是否为空或者只有一个元素
    if(head == null || head.next == null)
        return head;
    ListNode pre = null; //记录前一个节点
    ListNode current = head; //记录当前节点
    while (current != null) {
        //临时节点
        ListNode temp = current.next;
        //反转
        current.next = pre;
        pre = current;
        //进入下一个节点
        current = temp;
    }
    return pre;
}