牛客（15）反转链表

 //    题目描述
    //    输入一个链表，反转链表后，输出链表的所有元素。
    public static class ListNode {
        int val;
        ListNode next = null;

        ListNode(int val) {
            this.val = val;
        }
    }

    public static ListNode ReverseList(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode pre = null;
        ListNode now = null;
        while (head != null) {
            ListNode next = head.next;

            now = head;
            now.next = pre;
            pre = now;

            head = next;

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