I am currently attempting to answer the question Add Two Numbers from leetcode, my solution seems to function as I expect however in cases where my final carry int is needed it is not added to the end of my list in the final if statement.

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int carry = 0;
        ListNode* ans = l1;
        while(l1 || l2){
            if(l1 == nullptr) l1 = new ListNode();
            if(l1 == nullptr && l2){
                l1 -> val = (l2 -> val + carry) % 10;
                carry = (carry + l2 -> val) / 10;
                l2 = l2 -> next;
            }else if(l2 == nullptr && l1){
                int tempL1Val = l1 -> val;
                l1 -> val = (l1 -> val + carry) % 10;
                carry = (carry + tempL1Val) / 10;
            }else{
                int tempL1Val = l1 -> val;
                l1 -> val = (l1 -> val + l2 -> val + carry) % 10;
                carry = (tempL1Val + l2 -> val + carry) / 10;
                l2 = l2 -> next;
            }
            l1 = l1 -> next;
        }
        
        if(carry){
            l1 = new ListNode();
            l1 -> val = carry;
        } 
        return ans;
    }
};

Testcases where it fails:

l1 = [9,9,9,9,9,9,9]
l2 = [9,9,9,9]
output = [8,9,9,9,0,0,0]
expected_output = [8,9,9,9,0,0,0,1]

I have checked before and after and the new node does exist, however it is not being added to my list when I run the code.