AtCoder ABC #041 (Ruby)

abc041-a.rb

s = gets.chomp
i = gets.chomp.to_i
puts s[i-1]

abc041-b.rb

a, b, c = gets.chomp.split(" ").map(&:to_i)
f = 10**9 + 7
puts a * b * c % f

abc041-c.rb

n = gets.chomp.to_i
a = gets.chomp.split(" ").map(&:to_i)

a.zip((1..n).to_a).sort{|x, y| -(x[0] <=> y[0])}.each do |b|
  puts b[1]
end

abc041-d.rb

# http://abc041.contest.atcoder.jp/submissions/791292
# http://abc041.contest.atcoder.jp/submissions/790956
n, m = gets.chomp.split(" ").map(&:to_i)
x = []
y = []
m.times do
  line = gets.chomp.split(" ").map(&:to_i)
  x << line[0] - 1
  y << line[1] - 1
end

valid = []
dp = Array.new(1<<16, 0)
nn = (1<<n)

def contain(mask, pos)
  return !(mask & (1<<pos)).zero?
end

nn.times do |mask|
  valid[mask] = true
  m.times do |i|
    if contain(mask, y[i]) && !contain(mask, x[i]) then
      valid[mask] = false
      break
    end
  end
end

dp[0] = 1
nn.times do |mask|
  if valid[mask] then
    n.times do |i|
      if contain(mask, i) && valid[mask^(1<<i)] then
          dp[mask] += dp[mask^(1<<i)]
      end
    end
  end
end

puts dp[nn - 1]