Given a partially filled 9×9 2D array grid[9][9], the goal is to assign digits (from 1 to 9) to the empty cells (denoted by the number zero) so that every row, column, and subgrid of size 3×3 contains exactly one instance of the digits from 1 to 9.
Input:
[
[3, 0, 6, 5, 0, 8, 4, 9, 0],
[5, 2, 0, 0, 0, 0, 0, 0, 0],
[0, 8, 7, 0, 0, 0, 0, 3, 1],
[0, 0, 3, 0, 1, 0, 0, 8, 0],
[9, 0, 0, 8, 6, 3, 0, 0, 5],
[0, 5, 0, 0, 9, 0, 6, 0, 0],
[1, 3, 0, 0, 0, 0, 2, 5, 0],
[0, 0, 0, 0, 0, 0, 0, 7, 4],
[0, 0, 5, 2, 0, 6, 3, 0, 0]
]
Output: a valid Sudoku board