We can use a single set to store "seen" strings for each row, column, and box. For example, "5 in row 0", "5 in col 1", and "5 in box 0-0".

1. Initialize seen = set().
2. Iterate through r and c from 0 to 8:
   • If board[r][c] != '.':
     • val = board[r][c].
     • Create tokens: f"{val} in row {r}", f"{val} in col {c}", f"{val} in box {r//3}-{c//3}".
     • If any token is already in seen, return False.
     • Add all tokens to seen.
3. Return True.

• Time Complexity: O(1) (fixed 9x9 board).
• Space Complexity: O(1).

def is_valid_sudoku(board):
    seen = set()
    for r in range(9):
        for c in range(9):
            val = board[r][c]
            if val != '.':
                if (val, 'r', r) in seen or \
                   (val, 'c', c) in seen or \
                   (val, 'b', r//3, c//3) in seen:
                    return False
                seen.add((val, 'r', r))
                seen.add((val, 'c', c))
                seen.add((val, 'b', r//3, c//3))
    return True