题解 | 反转链表

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {

    public static void main(String[] args) {
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        node1.next = node2;
        node2.next = node3;

//        while (node1!=null){
//            System.out.println(node1.val);
//            node1 = node1.next;
//        }

        ListNode listNode = new Solution().ReverseList(node1);
        while(listNode!=null){
            System.out.println(listNode.val);
            listNode = listNode.next;
        }
    }
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    public ListNode ReverseList (ListNode head) {
        if (head==null){
            return null;
        }
        // write code here
        List<ListNode> list = new ArrayList<>();
        list.add(head);
        if(head.next!=null){
            ListNode nextNode = head.next;
            list.add(nextNode);
            while(nextNode!=null){
                list.add(nextNode);
                nextNode = nextNode.next;
            }
        }
        for(int i=list.size()-1;i>0;i--){
            list.get(i).next = list.get(i-1);
        }
        list.get(0).next = null;

        return list.get(list.size()-1);
    }
}