Valid Sudoku

Validate whether a partially filled Sudoku board is legal by checking every row, every column, and every 3x3 sub-grid for duplicate digits. Requirements Given a 9x9 Sudoku board, determine whether the filled cells obey Sudoku rules. You do not need to solve the puzzle; only validate the current board. Check all three constraints: No row contains the same digit twice. No column contains the same digit twice. No 3x3 sub-grid contains the same digit twice. Empty cells should be ignored. Notes Use sets for rows, columns, and boxes. The box index for cell (r, c) is (r // 3) * 3 + (c // 3). While scanning each non-empty cell, build a key for its row, column, and box; if any key was seen before, return false. Otherwise finish the scan and return true. Preparation Write the single-pass version with three arrays of sets. Be precise about what counts as empty input. Test duplicates in the same row, same column, same box, and a valid board with repeated digits in different boxes.