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**: - Each of the digits `1-9` must occur exactly once in each row. - Each of the digits `1-9` must occur exactly once in each column. - Each of the digits `1-9` must occur exactly once in each of the 9 `3x3` sub-boxes of the grid. The `'.'` character indicates empty cells.   **Example 1:** **Input:** board = [["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]] **Output:** [["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]] **Explanation:** The input board is shown above and the only valid solution is shown below: ```   **Constraints:** - `board.length == 9` - `board[i].length == 9` - `board[i][j]` is a digit or `'.'`. - It is **guaranteed** that the input board has only one solution.