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