// 37. Sudoku Solver
    public void solveSudoku(char[][] board) {
        
        helper(board);
    }
    
    private boolean helper(char[][] board) {

        for(int row = 0; row < 9; row++) {
            for(int col = 0; col < 9; col++) {
                if(board[row][col] == ‘.‘) {
                    for(char i = ‘1‘; i <= ‘9‘; i++) {
                        board[row][col] = i;
                        if(isValidSudoku(board, row, col) && helper(board))
                            return true;
                        // 还原
                        board[row][col] = ‘.‘;
                    }
                    return false;
                }
            }
        }
        return true;
    }

    public boolean isValidSudoku(char[][] board, int row, int col) {
        
        for(int i = 0; i < 9; i++) {
            if(i != col && board[row][i] == board[row][col])        // row
                return false;
            
            if(i != row && board[i][col] == board[row][col])          // colume
                return false;
            
            if(board[row / 3 * 3 + i / 3][col / 3 * 3 + i % 3] != ‘.‘ 
                    && board[row / 3 * 3 + i / 3][col / 3 * 3 + i % 3] == board[row][col])        // cube
                return false;
        }

        return true;
    }