Print Linked List Recursively Reversely

注意：可以不翻转链表，反着打印就行。

Solution 1: Iterative O(n); O(1)

先翻转，再打印

    public void printList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        while (cur != null) {
            ListNode next = cur.next; //cur.next can be null when cur is the last node
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        ListNode newHead = pre;
        while (newHead != null) {
//            if (newHead.next == null) {
//                System.out.print(newHead.val);
//                break;
//            }
//            System.out.print(newHead.val + "->");
            System.out.print(newHead.val + " ");
            newHead = newHead.next;
        }
    }

Solution 2: Iterative, stack O(n); O(n)

    public void printList(ListNode head){
        Stack<ListNode> stack = new Stack<>();
        while (head != null) {
            stack.push(head);
            head = head.next;
        }
        while (!stack.isEmpty())
            System.out.println(stack.pop().val);
    }

Solution 3: Recursive O(n); O(n)

    public void printList(ListNode head) {
        if (head == null) return;
        printList(head.next);
        System.out.print(head.val + " ");
    }