print_repeating.rb

def print_repeating(array)
 
  0.upto(array.size - 1) do |i| 
    flag = array[array[i].abs]

    flag = 1 if flag.nil?
    
    if flag >= 0
      array[array[i].abs] = -flag
    else
      puts array[i].abs
    end 
    
  end 
  
end

This algorithm works for any size integers, but it will only work in languages with dynamic arrays. Otherwise one could use a hash map with fixed memory size n.

If you know all of your integers are 0 <= x < n you don't need #6.