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.
5    8    7    
  1    2    5  
    2    3    1
8    9    1    
  3    5    7  
    1          5
4    6    3    
  7    3    9  
    9    8    6
** Moderate