lru_cache.rb

class LRUCache
  attr_reader :lookup, :size

  def initialize(size)
    @lookup = {}
    @size   = size
  end

  def get(key)
    return unless lookup[key]

    lookup[key] = lookup.delete(key)
  end

  def set(key, value)
    lookup.shift if lookup.size > size

    lookup.delete(key)
    lookup[key] = value
  end
end