Not like the previous solution here http://m.tkk7.com/chaocai/archive/2012/08/05/384844.html
The following solution not using the back tracing way is more concise and readable, but for the searching space becomes huger, the performance is much worser then the previous one.
(ns SICP.unit3)(defn conflictInCol? [s col]
(some #(= col %) s)
)
(defn conflictInDia? [s col]
(let [dia (count s)
n1 (fn [c] (Math/abs (- dia (.indexOf s c))))
n2 (fn [c] (Math/abs (- col c)))]
(some #(= (n1 %) (n2 %)) s)
)
)
(defn safe? [s col]
(not (or (conflictInCol? s col) (conflictInDia? s col)))
)
(defn next-level-queens [solutions-for-prev-level board-size current-level]
(let [solutions (atom [])]
(doseq [s solutions-for-prev-level]
(doseq [col (range 0 board-size)]
(if (safe? s col)
(reset! solutions (cons (conj s col) @solutions))
)
)
)
(if (< current-level (dec board-size))
(recur @solutions board-size (inc current-level))
(count @solutions)
)
)
)
(defn queens [board-size]
(next-level-queens (apply vector (map #(vector %) (range 0 board-size))) board-size 1)
)
Chao Cai (蔡超）
Sr. SDE
Amazon