working solution

test.rb

n = 5
lines = Array.new(n)

for i in 0..(n-1)
  lines[i] = 0
end

File.readlines("test").each do |item|
  item = item.to_i
  for i in 0..(n-1)
    if item > lines[i]
      lines[i] = item
      break
    end
  end
end

puts lines.sort