You have a faulty keyboard. Whenever you type a vowel on it (a,e,i,o,u,y), it reverses the string that you have written, instead of typing the character. Typing other characters works as expected. Given a string, return what will be on the screen after typing with your faulty keyboard.

faulty_keeb.ml

open Base

(*You have a faulty keyboard. Whenever you type a vowel on it (a,e,i,o,u,y), it reverses
  the string that you have written, instead of typing the character. Typing other
  characters works as expected. Given a string, return what will be on the screen after
  typing with your faulty keyboard.*)

let faulty_keyboard str =
  let rec aux acc = function
    | [] -> acc
    | el :: tl
      when Char.equal el 'a'
           || Char.equal el 'e'
           || Char.equal el 'i'
           || Char.equal el 'o'
           || Char.equal el 'u'
           || Char.equal el 'y' -> aux (List.rev acc) tl
    | el :: tl -> aux (el :: acc) tl
  in
  String.to_list str |> aux [] |> List.rev |> String.of_char_list
;;

let%test_module "faulty keyboard" =
  (module struct
    let%test_unit "test 1" = [%test_eq: string] (faulty_keyboard "string") "rtsng"

    let%test_unit "test 2" =
      [%test_eq: string] (faulty_keyboard "hello world!") "w hllrld!"
    ;;
  end)
;;