def conflict(state, nextX):
    nextY = len(state)
    for i in range(nextY):
        if abs(state[i] - nextX) in (0, nextY - i):
            return True
    return False

def queens(num, state=[]):
    for pos in range(num):
        if not conflict(state, pos):
            if len(state) == num-1:
                yield [pos]
            else:
                for result in queens(num, state +[pos]):
                    yield [pos] + result