SUDOKU
Write a number from 1 to 9 into each empty square in the grid so that every number from 1 to 9 appears exactly once in each horizontal row, each vertical column, and each 3×3 block.
3
6
8
5
9
4
7
2
6
2
3
7
1
7
9
3
5
6
1
9
3
6
7
8
4
**** Very hard