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Your list implementation also allows appending to the end.

To make this work efficiently, the list maintains a tail pointer to its last element. It is possible to do it without that pointer, but then you'd need to go through the whole list to find the last element everytime, making appending O(N) instead of O(1). For the same reason, your implementation also has a size counter.

When you add a new element to a previously empty list that tail pointer needs to be adjusted to point at the newly created single node.

Note that when adding to the front of the list, you only have to adjust the tail when the list was empty before. In all other cases, the tail stays where it was, and only the head and size change.

Try this,