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

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode answer = null;

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

        return answer;
    }
}