least_recently_set.rb

class LRS
  SIZE = 10
  attr_accessor :key_history, :inner

  def initialize()
    self.key_history = []
    self.inner = {}
  end
  
  def get(key)
    inner[key]
  end
  
  def set(key, value)
    inner[key] = value
    # If we have the key already, move it up
    if i = key_history.find_index(key)
      key_history.delete_at(i)
    end
    
    key_history << key
    
    if key_history.length > SIZE
      inner.delete(key_history[0])
      key_history.delete_at(0)
    end
  end
end