Reverse Linked List

两种做法，都要考虑好结尾处和怎么连接的

iteratively 

if(head ==null || head.next ==null) return head; 
        ListNode h  = new ListNode(-1);
        h.next = head;
        ListNode n = head.next;
        while(n!=null){
            ListNode t = n.next;
            n.next = h.next;
            h.next = n;
            head.next = t;  // must connect the dangling pointer to the new-end(head)
            n = t;
        }
        
        return h.next;

recursively

  // ref http://stackoverflow.com/questions/354875/reversing-a-linked-list-in-java-recursively 
    public ListNode reverseList(ListNode head) {
        if(head ==null || head.next ==null) return head; 

        ListNode secondH = head.next;
        ListNode newH = reverseList(secondH);
        head.next = null; //!!!! must
        secondH.next = head;
        return newH;