Method checkBlock

java/0036-valid-sudoku.java:49–72

(int idxI, int idxJ, char[][] board)

    }

    public boolean checkBlock(int idxI, int idxJ, char[][] board) {
        Set<Character> blockSet = new HashSet<>();
        //if idxI = 3 and indJ = 0
        //rows = 6 and cols = 3
        int rows = idxI + 3;
        int cols = idxJ + 3;
        //and because i initializes to idxI but only goes to rows, we loop 3 times (once for each row)
        for (int i = idxI; i < rows; i++) {
            //same for columns
            for (int j = idxJ; j < cols; j++) {
                if (board[i][j] == '.') {
                    continue;
                }
                
                if (blockSet.contains(board[i][j])) {
                    return false;
                }

                blockSet.add(board[i][j]);
            }
        }

        return true;
    }
}