From: Klaus Stein <usenet@...>
Date: 2009-01-15T21:09:44+09:00
Subject: Re: Returning a duplicate from an Array

Pierre Pat <theyojimbo@gmail.com> wrote:
> I'd do like this:
> a.select{|x| a.index(x) != a.rindex(x)}.uniq
> 
> Now, I wouldn't know which one is more efficient, but coding wise, I 
> like that way :-)
> 
If order is not important:
h = Hash.new(0)
a.each { |x| h[x]+=1 }
h.select {|k,v| v>1 }.keys

should be O(n) and with only one iteration through a.

Klaus
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