/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
#define NULL ((void *)0)

struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2){
    if (l1 == NULL)
        return l2;
    if (l2 == NULL)
        return l1;
    if (l1->val > l2->val){
        l2->next=mergeTwoLists(l1,l2->next);
        return l2;
    }
    else{
        l1->next=mergeTwoLists(l2,l1->next);
        return l1;
    }
}