This example shows a Python implementation of a Sudoku solver that finds a complete solution for a given 9x9 puzzle. The code defines a `Solution` class with a recursive backtracking algorithm in `solve_step`, which selects the cell with the fewest possible candidates and tries each valid number until the board is filled. It uses a `get_possible_numbers` method to compute remaining digits by checking row, column, and 3x3 square constraints. The solver modifies the board in place and is verified against an expected solution using test functions.