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The grid uses 0 to represent empty cells.
The isSafe() function ensures no duplicates in a row, column, or 3×3 box.
The solveSudoku() uses backtracking:
Tries a number 1–9 in each empty cell.
If a number works, moves to the next cell.
If stuck, it backtracks and tries a different number.
The final grid is printed as the solution.