blob: bd927589c10a12f64f5b5dd45ee29d123778b2f8 (
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#!/usr/bin/env ruby
input = $stdin.readlines.map do |i|
i =~ /Step ([A-Z]) must be finished before step ([A-Z]) can begin./
[$2, $1]
end
steps = input.flatten.uniq.sort
prereqs = {}
steps.each do |s|
prereqs[s] = []
end
input.each do |i|
prereqs[i.first] << i.last
end
order = []
until prereqs.empty? do
next_step = prereqs.select { |k,v| v.empty? }.keys.sort.first
order << next_step
prereqs.delete(next_step)
prereqs.each_key do |k|
prereqs[k].delete(next_step)
end
end
puts order.join
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