Valid Sudoku
Given a 9x9 board filled with digits or '.', determine whether it is a valid partially-filled Sudoku board.
Examples
Input: [["5","3",".","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8",".","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
Output: true
Input: [["8","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
Output: false
Hints
Use three separate sets to track seen digits in rows, columns, and 3x3 sub-boxes, ensuring no duplicates exist in any of them.
Iterate through each cell, and for non-empty cells, check if the digit already exists in the corresponding row, column, or sub-box set.
For sub-box validation, calculate the box index using integer division (`(row // 3) * 3 + (col // 3)`) to map cells to their respective 3x3 boxes.
Related Problems
Valid Sudoku
Given a 9x9 board filled with digits or '.', determine whether it is a valid partially-filled Sudoku board.