unsafe_key_lock.rb

require'set'
keys=('0000'..'9999').to_a
sets = Set.new keys
queue=[]
k = keys.first
sets.delete k
list = k.chars.map(&:to_i)
k = k[1..]
n = 0
loop do
  if n == 10
    k, n = queue.pop
    break unless k
    sets.add(k)
    k = k[...-1]
    list.pop
    next
  end
  k2 = k+n.to_s
  next n += 1 unless sets.include? k2
  list << n
  sets.delete k2
  if sets.empty?
    puts list.join
    binding.irb 
  end
  queue << [k2, n + 1]
  k = k[1..] + n.to_s
  n = 0
end