反转链表

public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class  {
    public ListNode ReverseList(ListNode head) {
        ListNode listNode = new ListNode(-1);
        while (head != null){
            ListNode next = head.next;
            head.next = listNode.next;
            listNode.next = head;
            head = next;
        }
        return listNode.next;
    }
}