Who owns the idea of ​​the test task?

Question

Android

I was interviewed at a company for the vacancy of Android-Junior, along the way I interviewed in other companies.
One of them sent a test task. Completed. Sent.
We made an appointment for a week later. During this time, a good offer was received from another company. I accepted it, but I liked the idea of ​​the test task. I took the idea for myself, redid it, corrected something and published it.
The question is how right is it to take an idea from a test task?

Answer 1

If you want this, and it was not paid, then there are no restrictions on its use.
And the idea does not belong to anyone: everyone can use it.

Answer 2

The idea cannot be taken away. You can take the job.
If Cyril and Methodius invented all the letters, this does not mean that they own all the books

Answer 3

Thank you!
Dispelled my mental anguish.