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    2    1    
        7    6  
    7    3    2
3    5    7    
  7    9    5  
    9    1    4
5    1    4    
  9    3    2  
    4    5    1
* Easy