My output have an extra 0 in it, why is that - c++

Question

So I'm doing practice on leetcode and this is the question:
You are given two non-empty linked lists representing two non-negative
integers. The digits are stored in reverse order and each of their nodes
contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the
number 0 itself.
Example:
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
And this is my solution:
class Solution {
public:
ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
vector<int> V1;
vector<int> V2;
int sum1 = 0;
int sum2 = 0;
ListNode *result = new ListNode(0);
auto l0 = result;
while(l1) {
V1.push_back(l1->val);
l1=l1->next;
}
for (auto it1 = V1.rbegin(); it1 != V1.rend(); it1++) {
sum1 = sum1 * 10 + (*it1);
}
while(l2) {
V2.push_back(l2->val);
l2=l2->next;
}
for (auto it2 = V2.rbegin(); it2 != V2.rend(); it2++) {
sum2 = sum2 * 10 + (*it2);
}
int sum3 = sum1 + sum2;
while (sum3 !=0) {
int extract = sum3 % 10;
l0->next = new ListNode(extract);
sum3 /= 10;
l0=l0->next;
}
return result;
}
};
And when I ran it, there is always and extra 0 in my output, for example:
Your input
[7,2,7] [2,4,2]
Your answer
[0,9,6,9]
Expected answer
[9,6,9]
I know there is a smarter way to solve this question, but I want to try to solve it in my way first

Answer 1

Its because you are creating the first node with 0. You have two solutions for that:
Skip the first element at the end of the function (workaround):
ListNode* aux = result;
result = result->next;
delete aux;
return result;
Not initialize the listnode to zero, use a nullpointer instead:
s
ListNode *result = nullptr;
// More code...
while (sum3 !=0) {
int extract = sum3 % 10;
if (l0 == nullptr) {
result = new ListNode(extract);
l0 = result;
}
else
l0->next = new ListNode(extract);
sum3 /= 10;
l0=l0->next;
}
Ofc, there are better solutions. You could do the sum directly, without using extra vectors/memory.

Answer 2

You are getting an extra zero because you are doing remainder and division operation 1 time extra than required. You have to modify the condition under last while loop.
while (sum3 > 9) {
int extract = sum3 % 10;
l0->next = new ListNode(extract);
sum3 /= 10;
l0=l0->next;
}