sets -- model solution

require 'set'

def find_duplicates(array)
  # write your code here
  set = Set.new
  duplicates = []

  array.each do |element|
    if set.include?(element)
      duplicates << element
    else
      set.add(element)
    end
  end

  duplicates
end

p find_duplicates([1, 2, 3, 1, 5, 6, 7, 8, 5, 2])
# => [1, 5, 2]

p find_duplicates([3, 501, 17, 23, -43, 67, 5, 888, -402, 235, 77, 99, 311, 1, -43])
# => [-43]