单链表翻转

    /*
     * three argvs: headNext(the head's next), tmp(move forward), node(the removed ListNode).
    */    
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode headNext = head.next;
        ListNode tmp = headNext;
        while (tmp.next != null) {
            ListNode node = tmp.next;
            tmp.next = node.next;
            head.next = node;
            node.next = headNext;
            
            headNext = head.next;
        }
        ListNode newHead = head.next;
        head.next = null;
        tmp.next = head;
        return newHead;
    }