Valid Sudoku
Given a 9×9 board, decide whether the filled cells are valid: no repeated digit in any row, any column, or any 3×3 sub-box. Empty cells are '.' and need no validation. The board does not have to be solvable — only the filled cells are checked.
The board arrives as nine strings of nine characters.
["53..7....", "6..195...", ".98....6.", "8...6...3", "4..8.3..1", "7...2...6", ".6....28.", "...419..5", "....8..79"] -> true
Hints
Hint 1
Keep nine sets each for rows, columns and boxes and make a single pass over the cells; the first digit already present in any of its three sets ends the check.
Hint 2
The box index is (row ÷ 3) × 3 + column ÷ 3 with integer division — derive it out loud, because getting it wrong fails silently.