Valid only a few Sudoku

Determine if a Sudoku is valid. The Sudoku board could be partially filled, where empty cells are filled with the character '.'.
[img]https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png[/img]
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.

Detecting whether a valid Sudoku, Sudoku rules we do not understand can go to Baidu. Here we only need to check the current status is valid, instead of finding a solution. So long as we check every row, every column, and if not are not repeated within nine small box numbers appear on it. code show as below:


public class Solution {
    public boolean isValidSudoku(char[][] board) {
        HashSet<Character> set = new HashSet<Character>();
        for(int i = 0; i < board.length; i++) {
            for(int j = 0; j < board[0].length; j++) {
                if(set.contains(board[i][j])) 
                    return false;
                else if(board[i][j] != '.')
                    set.add(board[i][j]);
            }
            set.clear();
        }

        for(int i = 0; i < board[0].length; i++) {
            for(int j = 0; j < board.length; j++) {
                if(set.contains(board[j][i]))
                    return false;
                else if(board[j][i] != '.')
                    set.add(board[j][i]);
            }
            set.clear();
        }

        for(int m = 0; m < 3; m++) {
            for(int n = 0; n < 3; n++) {
                for(int i = 3 * m; i < 3 * m + 3; i++) {
                    for(int j = 3 * n; j < 3 * n + 3; j++) {
                        if(set.contains(board[i][j]))
                            return false;
                        else if(board[i][j] != '.')
                            set.add(board[i][j]);
                    }
                }
                set.clear();
            }
        }
        return true;
    }
}

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