In description : return the first longest string consisting ......
It need to begin at the first longest string.

I tried to print the random test's arguments.
They are below:
array(3) {
[0]=>
string(11) "wlwsasphmxx"
[1]=>
string(23) "owiaxujylentrklctozmymu"
[2]=>
string(10) "wpgozvxxiu"
}
k is 2

Then it tell me :
Failed asserting that two strings are equal.
Expected: 'wlwsasphmxxowiaxujylentrklctozmymu'
Actual : 'owiaxujylentrklctozmymuwpgozvxxiu'

I don't know why it begin from "wlwsasphmxx"? "wlwsasphmxx" is not longest, right?

If there are many the same length strings in it.
['AAA','BB', 'C', 'DDD', 'EEE', 'FFFFFF']
K = 2

which one is it should return FFFFFFAAA or FFFFFFAAADDDEEE?