I'm working through a LeetCode problem with two linked lists that need to be added up.

It looks like my code is correctly generating a new list, with the expected nodes 7, and 9, followed by null, yet when I try to print out the results I'm running into an issue. I can see that the value of a->val is 7, but as soon as I get to the cout << a->val; line, it turns into another value.
[![enter image description here](https://i.stack.imgur.com/k45mR.png)](https://i.stack.imgur.com/k45mR.png)

[![enter image description here](https://i.stack.imgur.com/JjjWR.png)](https://i.stack.imgur.com/JjjWR.png)

Why is this happening?

```
using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode* next) : val(x), next(next) {}
};

ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
    ListNode ans;
    ListNode* ptr = &ans;
    int carry = 0;

    while (l1 || l2) {
        int a = 0;
        int b = 0;

        if (l1) {
            a = l1->val;
            l1 = l1->next;
        }

        if (l2) {
            b = l2->val;
            l2 = l2->next;
        }

        ptr->val = (a + b) % 10 + carry;
        carry = (a + b) / 10;

        if (l1 || l2) {
            ptr->next = new ListNode();
            ptr = ptr->next;
        }
    }

    return &ans;
}

int main()
{
    ListNode b(2);
    ListNode c(7);

    ListNode l1(2, &b);
    ListNode l2(5, &c);

    ListNode* a = addTwoNumbers(&l1, &l2);

    while (a) {
        cout << a->val;
        a = a->next;
    }
    cout << endl;
}

```