http://math.stackexchange.com/q/2012173/86776

balls.rb

# http://math.stackexchange.com/q/2012173/86776

def sim(n, runs)
  a0 = Array.new
  (1..n).each do |k|
    a0 << k
  end
  puts "a0 = #{a0}"
  h = Hash.new
  found = 0
  (1..runs).each do
    a = a0.clone
    ok = nil
    begin
      d = draw(n, a)
      ok = !consecutive(d)
      if not ok
        put_back(d, a)
      end
    end until ok
    d.sort!
    if h[d].nil?
      h[d] = true
      found += 1
    end
  end
  i = 0
  h.keys.each do |d|
    i += 1
    puts "#{i}. #{d}"
  end
  return found
end

def consecutive(d)
  if (d[0] - d[1]).abs == 1
    return true
  end
  if (d[0] - d[2]).abs == 1
    return true
  end
  if (d[1] - d[2]).abs == 1
    return true
  end
  return false
end

def draw(n, a)
  d = Array.new
  (1..3).each do
    begin
      i = rand(n)
    end until a[i] != 0
    d << a[i]
    a[i] = 0
  end
  return d
end

def put_back(d, a)
  a[d[0] - 1] = d[0]
  a[d[1] - 1] = d[1]
  a[d[2] - 1] = d[2]
end

sim(17, 10000)