反转链表

public ListNode ReverseList(ListNode head) {

        if (head == null)

            return null;

        ListNode dummy = new ListNode(-1);

        ListNode second = null;

        while (head != null) {

            second = head.next;

            head.next = dummy.next;

            dummy.next = head;

            head = second;

        }

        return dummy.next;

}