ruby tests seems to be broken.
the approved solution doesn't work for now

Thank you for the detailed explaination

I'm sure that I have to return 'p' answer from ["o", "q", "r", "s", "t", "u"]. But tests want me to return 'P' as I can see.
Random tests
Testing for ["b", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r"]
It should work for random inputs too - Expected: "C", instead got: "c"
Testing for ["g", "h", "i", "j", "k", "l", "m", "o", "p", "q", "r"]
It should work for random inputs too - Expected: "N", instead got: "n"
Testing for ["e", "f", "g", "h", "i", "j", "k", "l", "m", "o"]
It should work for random inputs too - Expected: "N", instead got: "n"
Testing for ["r", "s", "u", "v"]
It should work for random inputs too - Expected: "T", instead got: "t"
Testing for ["I", "J", "K", "L", "M", "N", "O", "Q"]
Testing for ["f", "h", "i", "j"]
It should work for random inputs too - Expected: "G", instead got: "g"
Testing for ["A", "B", "C", "D", "E", "F", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U"]
Testing for ["R", "S", "T", "V", "W", "X"]
Testing for ["j", "k", "m", "n", "o", "p", "q", "r", "s"]
It should work for random inputs too - Expected: "L", instead got: "l"
Testing for ["m", "n", "o", "p", "r", "s", "t", "u", "v", "w", "x"]
It should work for random inputs too - Expected: "Q", instead got: "q"
Testing for ["D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X"]
Testing for ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "p"]
It should work for random inputs too - Expected: "O", instead got: "o"
Testing for ["j", "l", "m"]
It should work for random inputs too - Expected: "K", instead got: "k"
Testing for ["p", "q", "r", "s", "t", "v", "w", "x"]
It should work for random inputs too - Expected: "U", instead got: "u"
Testing for ["K", "L", "M", "N", "O", "P", "Q", "R", "T", "U", "V", "W", "X"]
Testing for ["H", "J", "K", "L", "M", "N", "O"]
Testing for ["L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "W"]
Testing for ["E", "F", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W"]
Testing for ["U", "V", "X", "Y"]
Testing for ["I", "J", "L", "M", "N"]
Testing for ["J", "K", "L", "M", "N", "O", "P", "R", "S", "T", "U", "V", "W"]
Testing for ["j", "k", "l", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y"]
It should work for random inputs too - Expected: "M", instead got: "m"
Testing for ["u", "v", "w", "y"]
It should work for random inputs too - Expected: "X", instead got: "x"
Testing for ["R", "T", "U", "V", "W"]
Testing for ["t", "v"]
It should work for random inputs too - Expected: "U", instead got: "u"
Testing for ["U", "W"]
Testing for ["E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "P", "Q", "R", "S", "T", "U", "V", "W", "X"]
Testing for ["F", "H"]
Testing for ["h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "w"]
It should work for random inputs too - Expected: "V", instead got: "v"
Testing for ["g", "i", "j", "k", "l", "m"]
It should work for random inputs too - Expected: "H", instead got: "h"
Testing for ["M", "N", "P", "Q", "R"]
Testing for ["t", "u", "w", "x", "y", "z"]
It should work for random inputs too - Expected: "V", instead got: "v"
Testing for ["s", "t", "u", "v", "x", "y", "z"]
It should work for random inputs too - Expected: "W", instead got: "w"
Testing for ["T", "U", "W"]
Testing for ["n", "o", "q", "r", "s", "t", "u"]
It should work for random inputs too - Expected: "P", instead got: "p"
Testing for ["j", "k", "l", "n", "o", "p", "q"]
It should work for random inputs too - Expected: "M", instead got: "m"
Testing for ["Q", "R", "S", "U"]
Testing for ["D", "E", "F", "G", "H", "I", "J", "K", "L", "N", "O", "P", "Q", "R", "S", "T", "U"]
Testing for ["h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "u", "v", "w"]
It should work for random inputs too - Expected: "T", instead got: "t"
Testing for ["k", "l", "m", "n", "o", "p", "r", "s", "t", "u", "v", "w"]
It should work for random inputs too - Expected: "Q", instead got: "q"

in examples I see
['a','b','c','d','f'] -> 'e'
['O','Q','R','S'] -> 'P'

but I'm getting: Expected: "P" from ["o", "q", "r", "s", "t", "u"]=low-case array

I understood my problem: the algorithm wasn't optimal.
For now it works.

Process was terminated. It took longer than 12000ms to complete

what's wrong?

it takes 2 ms for basic tests to run
could anyone help me please?
Can I get some kind of debug info?

Testing for ["k", "l", "m", "n", "p", "q", "r", "s"]
It should work for random inputs too - Expected: "O", instead got: "o"
what I've done wrong?

please explain this:
Random tests
Testing for ["o", "q", "r", "s", "t", "u"]
It should work for random inputs too - Expected: "P", instead got: "p"
why should it return 'P' instead of 'p' ?