21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

含义：合并另个已经排好序的列表

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val)
        {
            l1.next = mergeTwoLists(l1.next,l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l1,l2.next);
            return l2;
        }      
    }