Find the first recurring character in a string

Questions & Help Questions
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2

Same way, probably.

defmodule Test do
  def first_recurring(str) do
    do_first_recurring(str, [])
  end

  defp do_first_recurring(<<letter, rest::bytes>>, chars) do
    if letter in chars do
      <<letter>>
    else
      do_first_recurring(rest, [letter | chars])
    end
  end
  defp do_first_recurring("", _chars), do: nil
end

Not particularly efficient, but should provide you with a general idea.

iex(2)> Test.first_recurring("ABCDAHWX")
"A"