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