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