def solve_sudoku(sudoku: list) -> list or None:
    """
    Function which solves sudoku
    :param sudoku: 2d list that represents sudoku, empty spaces should be marked as 0
    :return: returns solution or None
    """
    solved = sudoku.copy()
    empty_spaces = find_empty(sudoku)
    counter = 0
    correct = False
    while 0 <= counter < len(empty_spaces):
        i, j = empty_spaces[counter]
        for tested_value in range(solved[i][j] + 1, 10):
            correct = is_correct(tested_value, solved, (i, j))
            if correct:
                solved[i][j] = tested_value
                break
        if correct:
            counter += 1
        else:
            solved[i][j] = 0
            counter -= 1

    if correct:
        return solved