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Fill a 9×9 grid following Sudoku rules.
Problem: Fill empty cells so each row, column, and box has digits - exactly once.
Approach: For each empty cell, try digits -. Check validity (no duplicate in row, column, box). If valid, place and recurse. Backtrack if no digit works.
Time: worst case, constraints prune heavily.
Space: for recursion.
See the implementation unit for code.
