#2

public void fillInnRemainingOfBoard() throws Exception {
    if (next == null && legalValue(this.getNumericValue()))
        return;

    if (this instanceof PrefilledSquare)
        next.fillInnRemainingOfBoard();
    else {
        for (int value = 1; value <= row.getLength(); value++) {
            if (legalValue(value)) {
                value = value + "";
                illInnRemainingOfBoard();
            }
        }
        value = 0 + "";
    }
}