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.
5
6
7
3
6
4
7
9
9
2
4
7
8
1
3
4
7
1
4
9
8
1
3
4
6
*** Hard