Me too,I used Log10 because Math makes me goes YES

As it stands, it's testing your knowledge of algorithms. Using BigInt only tests your ability to call someone else's solution.

This is a 4kyu kata FFS.

You could take a .Distinct() of row, column, and 3x3 square to make sure you have 9 distinct values. This would also allow you to verify each number is within the 1-9 boundaries.