Odd Occurrences In Array

Elixir Odd Occurrences In Array

defmodule OddOccurrencesInArray do
  def odd_occurrences_in_array(a) do
    a
    |> Enum.reduce(%{}, fn v, count ->
      if Map.has_key?(count, v), do: Map.delete(count, v), else: Map.put(count, v, 1)
    end)
    |> Map.keys()
    |> List.first()
  end
end

This uses XOR to cancel out pairs, leaving only the value that appears an odd number of times.