Oops!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "list.h"

struct ListNode {

    int val;
    struct ListNode *next;
};

/*
 * [in]
 * @nd1: linked list of nodes
 * @nd2: linked list of nodes
 * 
 * [out]
 * none
 *
 * [in/out]
 * @carry(in): carry value of last sum
 * @carry(out): carry value of current sum
 */
static struct ListNode *addTwoNodes(struct ListNode *nd1,
                    struct ListNode *nd2,
                    int *carry)
{
    struct ListNode *nd_sum = malloc(sizeof(struct ListNode));

    if (!nd_sum) {
        printf("malloc node failed\n");
        return NULL;
    }
    nd_sum->val = 0;
    nd_sum->next = NULL;

    nd_sum->val = (nd1->val + nd2->val + *carry) % 10;
    *carry = (nd1->val + nd2->val + *carry) / 10;

    return nd_sum;
}

struct ListNode *addTwoNumbers(struct ListNode *l1, struct ListNode *l2)
{
    struct ListNode *nd1 = l1;
    struct ListNode *nd2 = l2;
    struct ListNode *nd_cont = NULL;
    struct ListNode nd_zero = { 0, NULL };

    struct ListNode *l_head = NULL;
    struct ListNode *l_tail = l_head;
    struct ListNode *nd_sum = NULL;
    int carry = 0;

    while (nd1 && nd2) {
        nd_sum = addTwoNodes(nd1, nd2, &carry);
        if (!nd_sum)
            return NULL;

        if (!l_head) { /* empty list */
            l_tail = l_head = nd_sum;
        } else { /* l_tail keeps track of the last node of list */
            l_tail->next = nd_sum;
            l_tail = nd_sum;
        }

        nd1 = nd1->next;
        nd2 = nd2->next;
    }

    if (!nd1 && !nd2) {
        goto carry;
    } else if (nd1 && !nd2) {
        nd_cont = nd1;
    } else { /* !nd1 && nd2 */
        nd_cont = nd2;
    }

    while (nd_cont) {
        nd_sum = addTwoNodes(nd_cont, &nd_zero, &carry);
        if (!nd_sum)
            return NULL;

        if (!l_head) { /* in case one of the two lists is initially empty */
            l_tail = l_head = nd_sum;
        } else {
            l_tail->next = nd_sum;
            l_tail = nd_sum;
        }

        nd_cont = nd_cont->next;
    }

carry:
    if (carry) {
        nd_sum = malloc(sizeof(struct ListNode));
        if (!nd_sum)
            return NULL;
        nd_sum->val = carry;
        nd_sum->next = NULL;

        l_tail->next = nd_sum;
        l_tail = nd_sum;
    }

    return l_head;
}