def n_queens(N, rows = None, arrangements = None):
    if rows is None:
        rows = N

    if arrangements is None:
        arrangements = []

    if rows == 0:
        return arrangements

    if rows == N:
        arrangements = [[]]

    new_arrangements = []
    for col in range(N):
        cell = N - rows, col
        # if `arrangements` is empty, then this list comprehension is also empty.
        # that's a problem for starting out the list of possible arrangements.
        # We therefore do a hack in L12 in order to populate the first row of
        # the board
        new_arrangements += [[cell] + arrangement for arrangement in arrangements
                        if no_conflict(cell, arrangement)]

    return n_queens(N, rows - 1, new_arrangements)


def no_conflict(cell, arrangement):
    for position in arrangement:
        if position[0] == cell[0] or position[1] == cell[1]:
            return False
        if abs(position[0] - cell[0]) == abs(position[1] - cell[1]):
            return False
    return True

if __name__ == '__main__':
    print(n_queens(0))
    print(n_queens(1))
    print(n_queens(2))
    print(n_queens(3))
    print(n_queens(4))
    print(len(n_queens(8)))

    for i in range(10):
        assert(all(len(arrangement) == i for arrangement in n_queens(i)))