class Array
  # swap index of a & b of caller array
  # pry>  [1, 2, 3, 4, 5].swap!(0, 4)
  # pry>  [5, 2, 3, 4, 1]
  def swap!(a, b)
    self[a], self[b] = self[b], self[a]
    self
  end
end