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