1def is_valid_sudoku(board):2    rows = defaultdict(set)3    cols = defaultdict(set)4    sq = defaultdict(set)56    for r in range(9):7        for c in range(9):8            if board[r][c] == ".":9                continue10            if board[r][c] in rows[r] or board[r][c] in cols[c] or board[r][c] in sq[(r // 3, c // 3)]:11                return False1213            rows[r].add(board[r][c])14            cols[c].add(board[r][c])15            sq[(r // 3, c // 3)].add(board[r][c])1617    return True