Takes a 9×9 2D array grid[9][9], and assigns 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.
Using Backtracking, and Bitmask.
#Complexity Analysis:
1)Time complexity: O(9^(nn)). For every unassigned index, there are 9 possible options so the time complexity is O(9^(nn)). The time complexity remains the same but checking if a number is safe to use is much faster, O(1). 2)Space Complexity: O(n*n). To store the output array a matrix is needed, and 3 extra arrays of size n are needed for the bitmasks.