You just have to know about substraction ;)

could be that list is too much? some set() related methods can work, but then, there are other approaches to the problem

Your solution must be printing stuff.

are you printing a ton of stuff?

How to overcome "Max Buffer Size Reached (1.5 MiB)"? Any suggestions on how to overcome it?

I think the solution is a lot simpler than most people think, I solved it and I have no clue what the triangular number sequence is.

Just looking through the discussions, lots of people seem to be going waaay off in the wrong direction.
I didn't have any trouble identifying the correct approach, but there's nothing in the description that suggests there's a trick to this.
As for the kyu, according to the wiki, level 6 is considered novice. Is it reasonable to assume that a novice would intuitively recognise the big O implications of a sub optimal solution here without any prompting? Well, look through the discussions.

Why? What does the kyu have to do with it? Anyway, my ignorance of any known sequence didn't prevent me from solving the kata.

Lookin' good!

Changed the description!
Please check.

Would be helpful if the description explicitly stated that the order of occurrence of words in the original string needed to be maintained in the result.

Other than that, nice kata!

Hmmm, you're right.
The test cases should probably pick up on that.