Uncover secrets

def uncover_secret(triplets)
  pairs = triplets.flat_map{|a,b,c|
    [[a,b], [a,c], [b,c]]
  }
  letters = pairs.flatten.uniq
  result = []
  until letters.empty?
    next_letter = letters.find{|l| !pairs.map(&:last).include?(l) }
    raise "FAIL" unless next_letter
    result << next_letter
    letters.delete next_letter
    pairs.delete_if{|a,b| a == next_letter}
  end
  result.join
end

secret1 = "whatisup"
triplets1 = [
  ['t','u','p'],
  ['w','h','i'],
  ['t','s','u'],
  ['a','t','s'],
  ['h','a','p'],
  ['t','i','s'],
  ['w','h','s']
]

p uncover_secret(triplets1)