36. Valid Sudoku

可以只loop一遍
https://discuss.leetcode.com/topic/9748/shared-my-concise-java-code

public boolean isValidSudoku(char[][] board) {
    for(int i = 0; i<9; i++){
        HashSet<Character> rows = new HashSet<Character>();
        HashSet<Character> columns = new HashSet<Character>();
        HashSet<Character> cube = new HashSet<Character>();
        for (int j = 0; j < 9;j++){
            if(board[i][j]!='.' && !rows.add(board[i][j]))
                return false;
            if(board[j][i]!='.' && !columns.add(board[j][i]))
                return false;
            int RowIndex = 3*(i/3)+j/3;
            int ColIndex = 3*(i%3)+j%3;
            if(board[RowIndex][ColIndex]!='.' && !cube.add(board[RowIndex ][ColIndex]))
                return false;
        }
    }
    return true;
}