open List
 
let rec safe (x1, y1) (x2, y2) =
  x1 <> x2 && y1 <> y2 && x2 - x1 <> y2 - y1 && x1 - y2 <> x2 - y1;;
 
let rec search f n qs ps accu = match ps with
  | [] -> if length qs = n then f qs accu else accu
  | q::ps -> search f n (q::qs) (filter (safe q) ps) (search f n qs ps accu)
 
let n = try int_of_string Sys.argv.(1) with _ -> 8
 
let rec init n f = if n=0 then [] else f(n-1) :: init (n-1) f
 
let ps = flatten (init n (fun i -> init n (fun j -> i, j)))
 
let print qs =
  let a = Array.init n (fun _ -> String.make n '.') in
  List.iter (fun (x, y) -> a.(y).[x] <- 'Q') qs;
  Array.iter print_endline a;;
 
let sols = search (fun qs -> print qs; (+) 1) n [] ps 0;;
 

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