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

def find_start(arr)
  flat = arr.flatten
  first, second = flat.reject { |val| flat.count(val) > 1}
  start_city = if flat.index(first) % 2 == 0 #if index of first is even
                 first
               else
                 second
               end
  arr.find { |pair| pair[0] == start_city }
end