solve the N queens problem using flat_map

queens.rb

require "pp"

def queens(n, k = n)
  if k == 0
    [[]]
  else
    queens(n, k - 1).flat_map {|queens|
      (1..n).map {|col| [k, col]}.select {|queen|
        queens.all? {|q|
          q[0] != queen[0] &&
            q[1] != queen[1] &&
            (q[0] - queen[0]).abs != (q[1] - queen[1]).abs
        }
      }.map {|queen| [queen, *queens]}
    }
  end
end

pp queens(8)