Unable to iterate to next ListNode, since the current node is getting overridden with the listnode.next value

Question

This is a leetcode problem.

I saw the solution however while trying to use my own logic, I cannot iterate over to the next node.

public ListNode AddTwoNumbers(ListNode l1, ListNode l2)
{
    try
    {
        string l1NodeData = null;
        string l2NodeData = null;

        do
        {
            l1NodeData = l1NodeData + l1.val;
            l1 = l1.next;
        } while (l1 != null);

        do
        {
            l2NodeData = l2NodeData + l2.val;
            l2 = l2.next;
        } while (l2 != null);

        int sum = Convert.ToInt32(l1NodeData) + Convert.ToInt32(l2NodeData);

        var sumInChar = sum.ToString().ToArray();

        ListNode dummyhead = new ListNode(0);
        ListNode ans = dummyhead;

        for (int i = sumInChar.Length - 1; i > 0; i--)
        {
            ans.val = Convert.ToInt32(sumInChar[i].ToString());
            ListNode tempListNode = new ListNode(Convert.ToInt32(sumInChar[i - 1].ToString()));
            ans.next = tempListNode;
            ans = ans.next;

            //here(ans = ans.next) after assigning the next node, the data of the current node is getting overridden.
        }

        return ans;
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
        throw;
    }
}

In the above solution where I am assigning ans = ans.next, there the current node is overridden with the next node, however, I just want to move to the next node in order to iterate.

Could someone please help me on this?

Answer 1

In the above solution where I am assigning ans = ans.next, there the current node is overridden with the next node, however, I just want to move to the next node in order to iterate.

The assignment is to a variable. That never mutates your linked list. This way of assigning to a variable is exactly what you need to traverse a linked list. It does not modify it. In order to mutate it, you will always need to assign to a member (property) of a node, like its next property.

In the code comments you wrote:

the data of the current node is getting overridden.

No, it looks like that because you return the reference to the last node that was created in the loop. Instead you should return dummyHead.next; This way you return the first node that was created in the loop.

In comments you wrote:

...I have not made any assignment to dummyhead right, then how does data reside in that variable?

You have referenced ans to the same node as dummyHead, and you did assign to ans.next, which in the first iteration of the loop is synonymous to dummyHead.next. With those assignments to ans.next the nodes get linked to eachother.

Note that dummyHead is an extra node that is created to ease the code in the loop -- not having to make a special case for creating the real head node. After the loop that dummyHead is then ignored (it served its purpose), and the real head (that is sitting right after it) is returned.