Why did you add List to the solution if you could just use HashSet and call ToList?

Adding a List to a solution, despite the possibility of using a HashSet with a subsequent call to ToList(), can be due to several reasons:

1. Preserving the order of elements. HashSet does not guarantee order, while List maintains the order of insertion. If order is important — List is necessary.

2. Performance during frequent index access. List provides fast access by index, HashSet does not.

3. Avoiding unnecessary conversions. If data is initially collected in a List and then converted to a HashSet to remove duplicates, then converting back to a List may be redundant.

Example:

HashSet<int> set = new HashSet<int> {1, 2, 3};
List<int> list = set.ToList(); // order is not guaranteed

List<int> listOrdered = new List<int> {1, 2, 3};
// if this specific order is needed, HashSet is not suitable

Thus, the choice of data structure depends on requirements for order, performance, and ease of use.