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