leetcode

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class  {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l2==NULL) return l1;
        if(l1==NULL) return l2;
        if(l1->val<l2->val){
            l1->next = mergeTwoLists(l1->next,l2);
            return l1;
        } else{
            l2->next = mergeTwoLists(l1,l2->next);
            return l2;
        }
    }
};