Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells. A sudoku solution must satisfy **all of the following rules**: 1. Each of the digits `1-9` must occur exactly once in each row. 2. Each of the digits `1-9` must occur exactly once in each column. 3. Each of the digits `1-9` must occur exactly once in each of the 9 `3x3` sub-boxes of the grid. The input is a 9x9 2D array where `0` represents an empty cell. Return the completed board.