ex.rb

def initial_letter(rules)
  lefts = []
  rights = []
  rules.each do |rule|
    left, right = rule.chars
    lefts << left
    rights << right
  end

  lefts.each do |chr|
    next if rights.include?(chr)
    return chr
  end
end


def find_follower(string, rules)
  predecessor = string.last
  next_rule = rules.find do |rule|
    rule.chars.first == predecessor
  end

  next_rule[-1]
end

def find_word(rules)
  # find the 1st letter
  # join other letters
  # voila
  initial = initial_letter(rules)
  string = [initial]

  rules.count.times do
    string << find_follower(string, rules)
  end

  string.join('')
end

puts find_word(["to", "je", "et", "on"]) # => jeton