The code demonstrate an elegance of Ruby with its object oriented approach to solve popular task about finding a celebrity among a group of persons.

find_celebrity.rb

# frozen_string_literal: true

class Array
  def except(value)
    self - [value]
  end
end

class Person
  attr_reader :name, :known_persons_names

  def initialize(name, known_persons_names = [])
    @name = name
    @known_persons_names = {}
    add_known_persons(known_persons_names) unless known_persons_names.empty?
  end

  def add_known_persons(known_persons_names)
    known_persons_names.each do |p|
      @known_persons_names[p] = true
    end
  end

  def knows?(person_name)
    @known_persons_names[person_name]
  end

  def doesnt_knows?(person_name)
    !knows?(person_name)
  end
end

class PersonGroup
  attr_reader :persons

  def initialize(persons = [])
    @persons = []
    add(persons) unless persons.empty?
  end

  def add(persons)
    persons.each do |person|
      @persons << person
    end
  end

  def find_celebrity_candidate
    l = 0
    h = @persons.count - 1
    while l != h
      if @persons[l].knows?(@persons[h].name)
        l += 1
      else
        h -= 1
      end
    end
    @persons[l]
  end

  def find_celebrity
    candidate = find_celebrity_candidate
    @persons.except(candidate).each do |other_person|
      return nil if candidate.knows?(other_person.name) || other_person.doesnt_knows?(candidate.name)
    end
    candidate
  end
end

my_group = PersonGroup.new(
  [
    Person.new('Kamil', %w[Ivan Alex Diman Madonna]),
    Person.new('Ivan', %w[Kamil Alex Diman Madonna]),
    Person.new('Alex', %w[Kamil Ivan Diman Madonna]),
    Person.new('Diman', %w[Kamil Ivan Alex Madonna]),
    Person.new('Madonna', [])
  ]
)

puts my_group.find_celebrity.inspect