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.
6    5    2    
  4    3    1  
    8    6    4
3    2    1    
  7    6    5  
    9    1    3
5    4          
  3    9    6  
    4    8    7
**** Very hard