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