But in O notation it will be 3x vs x² i think this is still better than having nested maps or foreach.

This solution only fits on a single line but forces the array to be traversed twice.

I found the solution, but my string is not really CSV lines with expected separator '\n'

In PHP I have this result :
Expected: '0,1,2,3,45\n10,11,12,13,14\n20,21,22,23,24\n30,31,32,33,34'
Actual : '0,1,2,3,45\n
10,11,12,13,14\n
20,21,22,23,24\n
30,31,32,33,34'

I just put '\n' in the end of line. But it's seems to bee not good

I don't understand.

I'm on PHP. In basic tests, when n strings = 6 and k = 6, my function returns '' and it is correct.

But in random tests, when n strings = 14 and k = 14, my function returns '' and it is asked for a string consisting of all the strings.

Idem for me (on PHP)

I was tempted to use the array_map(), max() and min() functions but with this solution each array is scanned 3 times. Which doesn't pose a problem, we're fine with small arrays.

I guess it's clearer with pictures :)

Fixed.

I think the issue is fixed, could you please verify?

It seems to be a bug, I'm not sure yet if it's a bug in the kata, or in setup of PHP on Codewars.

Thank you for your reply. And sorry for not specifying what language I'm using. :)