Hey there, I seem to be missing something.

Don't worry about brain meltdown - you are trying to do a very tricky thing here: You are modifying the list - including its size! - inside a for-loop that depends on the size of that very list. Don't get me wrong, I appreciate your logic: If I can remove an item, I will do it immediately, and then the index of the next item automatically drops by one and I can look at this item next. But consider the following questions: What happens if a lot of your items only contained "l"? Then you increase the list size and your index value would never catch up to the list size and the for-loop will go on forever. What happens if a lot of your items only contained "r" and you would have to remove all of them? Then your list size gets smaller and smaller, but the index value stays the same, maybe causing some elements near the end of your list not to be considered at all. What happens if a lot of your items contain both letters and should be left alone? You haven't covered that case at the moment, so the index-value would not increase and instead it would check the current index position forever. You can now try to fix this messy situation with a lot of incrementing/decrementing the index value, but my advice is: Create a second, empty ArrayList as your target/output-list. Iterate through every element of the original list with a "standard" for-loop. Whenever the element should be removed from the list, simply do not add it to the target list. Whenever it should be doubled, add it twice, whenever it should be left alone, just add it once. Finally, return the target list. Since modifying a list while iterating over it is such an intricate thing, there is a special concept called Iterator which you can use to accomplish these tasks. (You might need to read up on it, here's a CodeGym article covering the basics: https://codegym.cc/groups/posts/198-how-to-use-a-java-iterator-a-brief-tutorial . But for now you can always go with the "copy/clone/targetlist"-approach.