99 questions No.90

haskell_90.hs

generate :: Int -> [[Int]]
generate 0 = [[]]
generate n = iter [1..n] [] where
	iter [] ys = return (reverse ys)
	iter xs ys = do
		x <- xs
		iter (filter (/=x) xs) (x:ys)

queens :: Int -> [[Int]]
queens n = filter test $ generate n

test :: [Int] -> Bool
test [] = True
test (i:is) = not (check 1 i is) && test is

check :: Int -> Int -> [Int] -> Bool
check _ _ [] = False
check n j (i:is) =
	if j == i + n || j == i - n
	then True
	else check (n+1) j is