21. Merge Two Sorted Lists

 1 class Solution {
 2     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
 3         if(l1==null) return l2;
 4         if(l2==null) return l1;
 5         if(l1.val<l2.val) {
 6             l1.next = mergeTwoLists(l1.next,l2);
 7             return l1;
 8         }
 9         else {
10             l2.next = mergeTwoLists(l2.next,l1);
11             return l2;
12         }
13        
14     }
15     
16 }