Use a List.FilterByBoolMask

Find something to compare the list with to get a series of True/False (True = Level, False = not Level), and use this as the Mask for the List.Filter

In Dynamo, I've found things work better if you can organize lists to perform the same operation on the entire list. ex - use the List.FilterByBoolMask and perform separate operations on each of the output lists ("in" and "out"). If you need more control over iteration, you might want to look at Code Blocks (using DesignScript) or the Python Script node.

It also looks like the Top Constraint parameter may not be the most straightforward parameter - if you pass the output from GetParameterValueByName to Object.Type, you can see that you'll either get a System.String or a Revit.Elements.Level object.

If you require the original list in the original index order, it might look something like the image below.

Thanks cellersick and GMcDowellJr, the Object.Type word give me idea how to solve this.