合并2个排序链表--leecode

递归求解：

    public static ListNode mergeTwoListNode(ListNode l1, ListNode l2) {
        if (l1 == null || l2 == null) {
            return (l1 == null) ? l2 : l1;
        }
        if (l1.val <= l2.val) {
            l1.next = mergeTwoListNode(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoListNode(l1, l2.next);
            return l2;
        }
    }