题目链接：https://leetcode-cn.com/problems/reverse-linked-list/
题目如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        HashMap<Integer,ListNode> hash=new HashMap<Integer,ListNode>();
        int count=0;

        //将每个节点放入hashmap中，用count计数
        while(head!=null){
            hash.put(count++,head);
            head=head.next;
        } 

        //倒序放入新的链表中，空指针中依次放入节点
        ListNode list=new ListNode(0);
        ListNode pre=list;
        while(--count>=0){
            pre.next=hash.get(count);
            pre=pre.next;
        }
        pre.next=null;

        return list.next;
    }
}