Given a partially filled 9×9 2D array ‘grid[9][9]’, the goal is to assign digits (from 1 to 9) to the empty cells so that every row, column, and subgrid of size 3×3 contains exactly one instance of the digits from 1 to 9.

Example:

Input:
grid = { {3, 0, 6, 5, 0, 8, 4, 0, 0}, 
         {5, 2, 0, 0, 0, 0, 0, 0, 0}, 
         {0, 8, 7, 0, 0, 0, 0, 3, 1}, 
         {0, 0, 3, 0, 1, 0, 0, 8, 0}, 
         {9, 0, 0, 8, 6, 3, 0, 0, 5}, 
         {0, 5, 0, 0, 9, 0, 6, 0, 0}, 
         {1, 3, 0, 0, 0, 0, 2, 5, 0}, 
         {0, 0, 0, 0, 0, 0, 0, 7, 4}, 
         {0, 0, 5, 2, 0, 6, 3, 0, 0} }
Output:
          3 1 6 5 7 8 4 9 2
          5 2 9 1 3 4 7 6 8
          4 8 7 6 2 9 5 3 1
          2 6 3 4 1 5 9 8 7
          9 7 4 8 6 3 1 2 5
          8 5 1 7 9 2 6 4 3
          1 3 8 9 4 7 2 5 6
          6 9 2 3 5 1 8 7 4
          7 4 5 2 8 6 3 1 9
Explanation: Each row, column and 3*3 box of 
the output matrix contains unique numbers.