Hi, you should simply use round instead of ceil

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

Thank you for the detailed explaination

You're mutating the array, that's why the tests fail.

Paste your code, mark it as having spoiler content.

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"

I've tested Ruby version and it works as expected. Are you sure you're not mutating the array somehow?

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

Case sensitivity!

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' ?