https://leetcode.com/problems/add-two-numbers/submissions/1259689057

failed test case

//my code in c.
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
int64_t sum(struct ListNode *root)
{
if(root==NULL)
{
    return 0;
}
return (sum(root->next)*10) + root->val;
}
struct ListNode* generateNode(int num)
{
    struct ListNode *p;
    p=(struct ListNode*)malloc(sizeof(struct ListNode));
    if(num<=0)
    {
        return NULL;
    }
    p->val=num%10;
    p->next=generateNode(num/10);
    return p;
    
}


struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    int64_t a1,a2;
    a1=sum(l1);
    a2=sum(l2);
    //printf("%d",a1+a2);
    struct ListNode *p;
    p=(struct ListNode*)malloc(sizeof(struct ListNode));
    p->val=0;
    p->next=NULL;
    if(a1+a2!=0){
    p=generateNode(a1+a2);
    }
    return p;

}

I the above is a code I wrote.
I think the error is due lack of storage space.
I tried my best if you know the answer please share me.