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.
  21      39  
43          1  
8      7      6
      7  5      
    8      9    
      6  2      
9      4      8
18          4  
    6      5  3
** Moderate