Valid Sudoku

class Solution {
    private int findAreaNumber(int i, int j) {
        if (i >= 0 && i < 3 && j >= 0 && j < 3) {
            return 1;
        } else if (i >= 0 && i < 3 && j >= 3 && j < 6) {
            return 2;
        } else if (i >= 0 && i < 3 && j >= 6 && j < 9) {
            return 3;
        } else if (i >= 3 && i < 6 && j >= 0 && j < 3) {
            return 4;
        } else if (i >= 3 && i < 6 && j >= 3 && j < 6) {
            return 5;
        } else if (i >= 3 && i < 6 && j >= 6 && j < 9) {
            return 6;
        } else if (i >= 6 && i < 9 && j >= 0 && j < 3) {
            return 7;
        } else if (i >= 6 && i < 9 && j >= 3 && j < 6) {
            return 8;
        } else if (i >= 6 && i < 9 && j >= 6 && j < 9) {
            return 9;
        } else {
            return 0;
        }
    }
    
    public boolean isValidSudoku(char[][] board) {
        HashMap<Integer, ArrayList<Integer>> rows = new HashMap<>();
        HashMap<Integer, ArrayList<Integer>> columns = new HashMap<>();
        HashMap<Integer, ArrayList<Integer>> areas = new HashMap<>();

        for (int i=0; i<9; i++) {
            for (int j=0; j<9; j++) {
                    if (board[i][j] == '.') { continue; }

                int number = Integer.valueOf(String.valueOf(board[i][j]));

                ArrayList<Integer> currentRows = rows.get(i + 1) == null ? new ArrayList<Integer>() : rows.get(i + 1);
                ArrayList<Integer> currentColumns = columns.get(j + 1) == null ? new ArrayList<Integer>() : columns.get(j + 1);

                // row check
                if (currentRows.contains(number)) {
                    return false;
                } else {
                    currentRows.add(number);
                    rows.put(i + 1, currentRows);
                }

                // column check
                if (currentColumns.contains(number)) {
                    return false;
                } else {
                    currentColumns.add(number);
                    columns.put(j + 1, currentColumns);
                }
            }
        }

        // check area
        for (int i=0; i<9; i=i+3) {
            for (int j=0; j<9; j=j+3) {

                for (int r=0; r<3; r++) {
                    for (int c=0; c<3; c++) {
                        if (board[i+r][j+c] == '.') { continue; }
                        
                        int number = Integer.valueOf(String.valueOf(board[i+r][j+c]));
                        int areaNumber = findAreaNumber(i + r, j + c);
                        ArrayList<Integer> currentArea = areas.get(areaNumber) == null ? new ArrayList<Integer>() : areas.get(areaNumber);

                        if (currentArea.contains(number)) {
                            return false;
                        } else {
                            currentArea.add(number);
                            areas.put(areaNumber, currentArea);
                        }
                    }
                }
            }
        }

        return true;
    }
}