omg

nice move dude

Very clever! haha

is O(n^2) here ?

But there is no "Z"

Why is it clever?

Very efficient and easy on the eyes, but this code fails case-sensitivity (even though the challenge didn't really specify.) :)

I knew there would be a way to do this with replaces, clever

basically did the same but just using the lowercase

you could just do return (condition_in_if_statement);

Good point, but you are basically using the same approach. Hence the time complexity of your solution isn't perfect as well.

Clever and simple, I think it shortened by replacing them with lowercase values and just call to uppercase at end.

can u explain the return statement

I did something very similar. Only difference is I mapped all 4 of "T", "A", "C", and "G" to different characters before doing another 4 "replaceAll()"s. So this runs slightly faster than mine. Good job!

At the return statment we could also do:
return builder.substring(0, builder.length() - 1);