Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

A partially filled sudoku which is valid.

 

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

 

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        Set<Character> set = new HashSet<Character>();
        for (int i = 0; i < board.length; i++) {
        	set.clear();
        	for (int j = 0; j < board.length; j++) {
        		if (board[i][j]!='.' && !set.add(board[i][j])) {
        			return false;
        		}
        	}
        }
        for (int i = 0; i < board.length; i++) {
        	set.clear();
        	for (int j = 0; j < board.length; j++) {
        		if (board[j][i]!='.' && !set.add(board[j][i])) {
        			return false;
        		}
        	}
        }
        for (int i = 0; i < board.length/3; i++) {
        	for (int j = 0; j < board.length/3; j++) {
        		set.clear();
        		for (int a = i*3; a < i*3+3; a++) {
        			for (int b = j*3; b < j*3+3; b++) {
        				if (board[a][b]!='.' && !set.add(board[a][b])) {
                			return false;
                		}
        			}
        		}
        	}
        }
        return true;
    }
}