Then there are two problems
a1 = ['te st'], a2 =['te', 'st']

You probably have a quadratic time solution !
You can solve this only looping through it once and have it be linear time.

About two months ago, I had no idea how to code. I cannot believe that I solved this today. Excellent kata!

how do you even get it to work for this such big numbers. my code only works for smaller numbers. perhaps i will really need a spoiler on that. i will be glad if you can put me through.

you're right, guess I need to improve my reading skills lol

In kata description it is written:

[(a, b), ...] or [[a, b], ...] or {{a, b}, ...} or ... will be sorted in increasing order of the "a".

So you need to sort your returned array according to first element, yes.

After I've solved it, I can't believe I hate math even more, whoa...

Argument mutation is not actually "Best Practices" though this solution has smart tricks

The problem here is that it will join array2 on every iteration...

genius!

nope

Hm, looks like you can hack any test cases in this platform

I tried this kind of stuff, but luck of experience in JVM make it impossible for me. I was like 20% close to this solution xD

No. Once is enough.

It's PRELOADED, you don't have to write it in your code omg...