LeetCode 2. Add Two Numbers

class Solution
{
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2)
{
if (l1 == NULL) return l2;
if (l2 == NULL) return l1;

ListNode *l3, *ans;
l3 = (ListNode*)malloc(sizeof(ListNode));
l3 = NULL;
ans = NULL;

int s = 0;

while (l1 != NULL && l2 != NULL)
{
int sum = l1->val + l2->val;
l1 = l1->next;
l2 = l2->next;

ListNode *tmp;
tmp = (ListNode*)malloc(sizeof(ListNode));
tmp->next = NULL;
tmp->val = (sum + s) % 10;
s = (sum + s) / 10;

if (ans == NULL)
{
ans = l3 = tmp;
}
else
{
l3->next = tmp;
l3 = l3->next;
}
}

while (l1 != NULL)
{
int sum = l1->val;
l1 = l1->next;

ListNode *tmp;
tmp = (ListNode*)malloc(sizeof(ListNode));
tmp->next = NULL;
tmp->val = (sum + s) % 10;
s = (sum + s) / 10;

l3->next = tmp;
l3 = l3->next;
}

while (l2 != NULL)
{
int sum = l2->val;
l2 = l2->next;

ListNode *tmp;
tmp = (ListNode*)malloc(sizeof(ListNode));
tmp->next = NULL;
tmp->val = (sum + s) % 10;
s = (sum + s) / 10;

l3->next = tmp;
l3 = l3->next;
}

if (s != 0)
{
ListNode *tmp;
tmp = (ListNode*)malloc(sizeof(ListNode));
tmp->next = NULL;
tmp->val = s;
s = 0;
l3->next = tmp;
l3 = l3->next;
}
return ans;
}
};