It doesn't matter if it's trivial, the users can't understand which input is valid and which is not. And no, you can't derive from the Kata's instructions that the string should ONLY contain those characters.

The description states, "Your function will take an input string that may contain any of the symbols "()" "{}" or "[]" to create groups."

In other words, the string will only contain those characters. It's trivial to make a single passthrough to remove any other characters, so I didn't think that it added anything to the problem to include them.

Most of your calculations are useless, look carefully at the drawing and use the hint.