#1

public void solveWithout(int row, int col) {
    if(row > squares.length - 1) {
        solutions.insert(squares);
        return;
    }

    if(squares[row][col] instanceof PrefilledSquare)
        nextWithout(row, col);
    else {
        for (int value = 1; value <= squares.length; value++) {
            if(squares[row][col].legalValue(value)) {
                squares[row][col].setValue(value + "");
                nextWithout(row, col);
            }
        }
        squares[row][col].setValue(0 + "");
       }
    }


public void nextWithout(int row, int col) {
    if(col < squares.length - 1)
        solveWithout(row, ++col);
    else
        solveWithout(++row, 0);
}