Valid Sudoku.java

/*
    Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

    Each row must contain the digits 1-9 without repetition.
    Each column must contain the digits 1-9 without repetition.
    Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.

    Note:
    A Sudoku board (partially filled) could be valid but is not necessarily solvable.
    Only the filled cells need to be validated according to the mentioned rules.
    
    Input: 
    board = 
    [["5","3",".",".","7",".",".",".","."]
    ,["6",".",".","1","9","5",".",".","."]
    ,[".","9","8",".",".",".",".","6","."]
    ,["8",".",".",".","6",".",".",".","3"]
    ,["4",".",".","8",".","3",".",".","1"]
    ,["7",".",".",".","2",".",".",".","6"]
    ,[".","6",".",".",".",".","2","8","."]
    ,[".",".",".","4","1","9",".",".","5"]
    ,[".",".",".",".","8",".",".","7","9"]]
    Output: true
*/

public boolean isValidSudoku(char[][] board) {
    Set seen = new HashSet();
    for (int i=0; i<9; ++i) {
        for (int j=0; j<9; ++j) {
            char number = board[i][j];
            if (number != '.')
                if (!seen.add(number + " in row " + i) ||
                    !seen.add(number + " in column " + j) ||
                    !seen.add(number + " in block " + i/3 + "-" + j/3))
                    return false;
        }
    }
    return true;
}