Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

Here's a possible solution in ruby without using Array#detect (as in this answer). Using Array#detect makes it too easy, I think.

ALPHABET = %w(a b c d e f g h i j k l m n o p q r s t u v w x y z)

def fnr(s)
  unseen_chars    = ALPHABET.dup
  seen_once_chars = []
  s.each_char do |c|
    if unseen_chars.include?(c)
      unseen_chars.delete(c)
      seen_once_chars << c
    elsif seen_once_chars.include?(c)
      seen_once_chars.delete(c)
    end
  end

  seen_once_chars.first
end

Seems to work for some simple examples:

fnr "abcdabcegghh"
# => "d"

fnr "abababababababaqababa"                                    
=> "q"

Suggestions and corrections are very much appreciated!