String Builder vs Lists

Question

I am reading in multiple files in with millions of lines and I am creating a list of all line numbers that have a specific issue. For example if a specific field is left bla

Answer 1

Is your output supposed to be human readable? If so, you'll hit the limit of what is reasonable to read, long before you have any performance/memory issues from your data structure. Use whatever is easiest for you to work with.

If the output is supposed to be machine readable, then that output might suggest an appropriate data structure.