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