def median(string)                           
  array = string.split(//) 
  case
  when array.length == 1  
    return array[0]
  when array.length == 2
    return array[0] + array[1]                
  when array.length.odd? && array.length >= 3                   
    return array[(array.length - 1) / 2] 
  when array.length.even? && array.length >= 4              
    return array[((array.length / 2 ) - 1)] + array[(array.length / 2)]
  else nil
  end
end
 
 
 
puts median("testing")
puts median("abc")
puts median("abcdef")

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