// 思路：迭代
class Solution {
    public ListNode reverseList(ListNode head) {
        // 判断链表是否为空或者只有一个结点
        if(head == null || head.next == null){
            return head;
        }

        ListNode cur = head;
        ListNode next = null;
        ListNode reverseHead = new ListNode(0);
        while(cur != null){
            next = cur.next;
            cur.next = reverseHead.next;
            reverseHead.next = cur;
            cur = next; //注意将 cur 后移
        }

        return reverseHead.next;
    }
}