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.

Solution:

对于每一行，每一列，每个九宫格进行验证，总共需要27次验证，每次看九个元素。

时间复杂度就是O(3*n^2), n=9。

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        if(board == null || board.length != 9 || board[0].length != 9)
            return false;
        
        for(int i = 0; i < 9; i++) {  //check each row
            HashSet map = new HashSet();
            for(int j = 0; j < 9; j++) {
                if(!map.add(board[i][j]) && board[i][j] != '.') {
                    return false;
                }
            }
        }
        
        for(int i = 0; i < 9; i++) {  //check each column
            HashSet map = new HashSet();
            for(int j = 0; j < 9; j++) {
                if(!map.add(board[j][i]) && board[j][i] != '.') {
                    return false;
                }
            }
        }
        
        for(int i = 0; i < 3; i++) {  //check each 9 sub-boxes
            for(int j = 0; j < 3; j++) {
                HashSet map = new HashSet();
                for(int m = i * 3; m < i * 3 + 3; m++) {
                    for(int n = j * 3; n < j * 3 + 3; n++) {
                        if(!map.add(board[m][n]) && board[m][n] != '.') {
                            return false;    
                        }
                    }
                }
            }
        }
        return true;
    }
}