Find a unique element in a compound array

Question

I am trying to solve a problem where I need to find the airport code in an array of arrays of that represents the starting point of a multi-city flight plan. For example: Given

Answer 1

trips = [['LAX', 'BWI'], ['BOS', 'SEA'], ['HNL', 'LAX'], ['SEA', 'HNL']]

arrivals = trips.map(&:last)

p trips.find{|fligth| ! arrivals.include? fligth[0] } #=> ["BOS", "SEA"]