206. Reverse Linked List

    // iteratively
    public ListNode reverseList(ListNode head) {
        
        if(head == null)
            return null;
        
        ListNode pre = head, cur = head.next;
        pre.next = null;
        while(cur != null) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        
        return pre;
    }

    public ListNode reverseList3(ListNode head) {
        return reverseListHelper(head, null);
    }
    private ListNode reverseListHelper(ListNode head, ListNode newHead) {
            
        if(head == null)
            return newHead;
        ListNode next = head.next;
        head.next = newHead;
        return reverseListHelper(next, head);
    }