public boolean isValidSudoku(char[][] board) {
		if(board == null || board.length != 9 || board[0].length != 9) return false;
		for(int i = 0; i < 9; i++) {
			if(!checkRowCol(board, i, true)) return false;
			if(!checkRowCol(board, i, false)) return false;
		}
		for(int i = 1; i <= 7; i += 3) 
			for(int j = 1; j <= 7; j += 3) {
				if(!checkBox(board, i, j)) return false;
			}
		return true;
	}