题目链接
迭代

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null,cur = head,next = null;
        while(cur!=null){
             //将cur.next存起来
             next = cur.next;
             //cur.next指向pre
             cur.next = pre;
             //将当前节点赋给pre
             pre = cur;
             //将当前节点的下一个节点赋给当前节点
             cur = next;
        }
        //返回当前节点
        return pre;
    }
}