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