题目描述：

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

输入：

head = [1,2,3,4,5]

输出：

[5,4,3,2,1]

代码实现：

1.根据题意创建一个结点类：

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;
    }
}

2.反转链表具体实现:

public class Main{
    public static void main(String[] args) {
        //创建一个简单的链表: 1 -> 2 -> 3
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        //反转操作
        ListNode res = reverseList(head);
        //查看结果
        ListNode now = res;
        while (now != null) {
            if (now.next != null) {
                System.out.print(now.val + "->");
            } else {
                System.out.print(now.val);
            }
            now = now.next;
        }//3->2->1
    }

    public static ListNode reverseList(ListNode head) {
        //前驱指针
        ListNode pre = null;
        //当前指针
        ListNode current = head;
        //后继指针
        ListNode next;
        //反转操作
        while (current != null) {
            //保存后继结点信息
            next = current.next;
            //当前结点指向前驱
            current.next = pre;
            //pre指向当前结点
            pre = current;
            //当前结点指针指向下一个结点
            current = next;
        }
        //pre为反转之后的头结点
        return pre;
    }
}