NP-hard just speaks about the algorithm complexity. The input size of a typical sudoku puzzles so small that even the most naive algorithm can do it quickly.
Incorrect. I tried the most naive algorithm when I was about 9. It generated every 9x9 grid of digits, checked if it was a Sudoku solution, and then it it matched the puzzle. I gave up while every row but the first was still full of 0s.
