public ListNode ReverseList(ListNode head) {

               if (head == null) {

            return null;

        }

        if(head.next==null){

            return head;

        }

        ListNode preNode = null;

        ListNode currentNode = head;

        ListNode afterNode = head.next;

        while (currentNode!=null){

            currentNode.next=preNode;

            preNode = currentNode;

            if (afterNode.next == null) {

                afterNode.next=currentNode;

                break;

            }

            currentNode=afterNode;

            afterNode = afterNode.next;

        }

        return afterNode;

    }