Given an array of integers where every integer occurs three times except for one integer, which only occurs once, find and return the non-duplicated integer. For example, given [6, 1, 3, 3, 3, 6, 6], return 1. Given [13, 19, 13, 13], return 19.

daily-coding-problem-40.rb

#!/usr/bin/env ruby

def non_duplicate_in_set set 
    freq = set.inject(Hash.new(0)) {|hash,word| hash[word] += 1; hash }
    freq.keys.each { |k| return k if freq[k] < 3 }
end 

set = [6, 1, 3, 3, 3, 6, 6]
puts non_duplicate_in_set(set)