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