class Solution {
    public void solveSudoku(char[][] board) {
        if (board == null || board.length == 0) return;

        solve(board);
    }

    private boolean solve(char[][] board){
        int n = board.length, m = board[0].length;

        for (int i = 0; i < n; i++){
            for (int j = 0; j < m; j++){
                if (board[i][j] == '.'){

                    for (char c = '1'; c <= '9'; c++){
                        if (valid(board, i, j, c)){
                            board[i][j] = c;    
                            if (solve(board)){
                                return true;
                            }
                            else{
                                // otherwise go back
                                board[i][j] = '.';
                            }
                        }
                    }
                    //if tried 1-9, but still cannot fill this blank, then return false
                    return false;
                }
            }
        }

        return true;
    }

    private boolean valid(char[][] board, int row, int col, char c){
        //row
        for (int i = 0; i < 9; i++){
            if (board[row][i] == c) return false;
        }
        //col
        for (int i = 0; i < 9; i++){
            if (board[i][col] == c) return false;
        }
        //cell
        for (int i = 0; i < 9; i++){
            if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == c) return false;
        }

        return true;
    }
}