class Solution {
    public static void main(String[] args) {
        Solution o = new Solution();
        int[] arr = {1,2,3,4};
        ListNode head1 = o.create(arr);
        System.out.println("链表：");
        o.print(head1);
        System.out.println();

        ListNode head2 = o.reverse(head1);
        System.out.println("链表反转后：");
        o.print(head2);
    }
    
    //链表反转
    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        ListNode next = head;
        while(next != null){
            next = curr.next; //这个必须放前面，如果放后面，curr.next就已经被更改为prev了，就丢掉了。

            curr.next = prev;
            
            prev = curr;
            curr = next;
        }

        //最后一个curr是null，prev才是组后一个元素的地址，也就是新链表的头结点。
        return prev;
    }
　　
　　//根据数组创建链表　
    public ListNode create(int[] arr){
        ListNode head = new ListNode(arr[0]);
        ListNode curr = head;
        for(int i = 1;i<arr.length;i++){
            ListNode tmp = new ListNode(arr[i]);
            curr.next = tmp;
            tmp.next = null;
            curr = tmp;
        }
        return head;
    }
　　
　　//给定链表头节点，遍历输出链表
    public void print(ListNode head){
        ListNode curr = head;
        while(curr != null){
            System.out.print(curr.val+" ");
            curr = curr.next;
        }
    }
}

//链表定义
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}