Complete a Uniquely Solvable Sudoku Board in Place

### Problem Complete the empty cells of a 9-by-9 Sudoku board. Each row, each column, and each of the nine 3-by-3 boxes must contain every digit from 1 through 9 exactly once. A period `'.'` marks an unfilled cell; existing digits must remain in place. The given board has exactly one solution. Change the supplied character matrix through this original interface; the function returns nothing: ```cpp void solveSudoku(vector<vector<char>>& board); ``` In the examples, each quoted row is shorthand for nine character cells of the matrix. ### Examples ```text Input rows: .23456789 4.6789123 78.123456 234.67891 5678.1234 89123.567 345678.12 6789123.5 91234567. Rows after the call: 123456789 456789123 789123456 234567891 567891234 891234567 345678912 678912345 912345678 ``` Every row of this input has only its diagonal cell missing, so its missing digit is forced. ```text Input rows: 123456789 456789123 789123456 234567891 567891234 891234567 345678912 678912345 912345678 Rows after the call: unchanged ``` ### Constraints - The board has exactly 9 rows, each containing 9 cells. - Every cell is a digit character or `'.'`. - The input has exactly one valid completion.