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