We are given a nxn partially solved square board and have to solve it according to the following rules
- Empty cells of the grid must be filled with numbers from 1-n
- Each row must have each number from 1-n exactly once
- Each column must have each number from 1-n exactly once
- Each of the n non-overlapping sub-grids of size sqrt(n) * sqrt(n) must have each number from 1-n exactly once
- n is a perfect square and sqrt() used above refers to the square root function