day12

class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(l1 == NULL) return l2;
 else if (l2 == NULL) return l1;
else if (l1->val < l2->val) {
l1->next = mergeTwoLists(l1->next, l2);
 return l1;
        } else {
l2->next = mergeTwoLists(l1, l2->next);
  return l2;
        }
   }
};