solve n_queen problem

n-queen.clj

(use '[clojure.contrib.combinatorics :only (permutations)])

(defn- valid? [board]
  {:pre [(vector? board)]}
  (let [n (count board)]
    (every?
     #(let [[i j] %] (and (not= (board i) (board j))
                          (not= (- i (board i)) (- j (board j)))
                          (not= (+ i (board i)) (+ j (board j)))))
     (for [i (range n) j (range n) :when (not= i j)] (vector i j)))))

(defn n-queen [n]
  {pre [(number? n)]}
  (for [board (permutations (range n))
        :when (valid? (vec board))]
    (vec board)))