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