From: Brian Candler <b.candler@...>
Date: 2008-10-01T16:34:47+09:00
Subject: Re: Mode method for Array

Shame that the standard Hash#invert doesn't handle duplicate values 
well. My suggestion:

class Hash
  def ninvert
    inject({}) { |h,(k,v)| (h[v] ||= []) << k; h }
  end
end

class Array
  def get_mode
    (inject(Hash.new(0)) { |h,e| h[e] += 1; h }.ninvert.max || 
[[]]).last
  end
end

p [3, 1, 1, 55, 55].get_mode
p [3, 1, 1, 55].get_mode
p [:foo, 3, "bar", :foo, 4, "bar"].get_mode
p [].get_mode

(with ruby 1.8 if there are multiple mode values you get them in an 
arbitary order; I think with 1.9 you'd get them in the order first seen 
in the original array)
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