<p>Determine if a <code>9 x 9</code> Sudoku board is valid. Only the filled cells need to be validated <strong>according to the following rules</strong>:</p> <ol> <li>Each row must contain the digits <code>1-9</code> without repetition.</li> <li>Each column must contain the digits <code>1-9</code> without repetition.</li> <li>Each of the nine <code>3 x 3</code> sub-boxes of the grid must contain the digits <code>1-9</code> without repetition.</li> </ol> <p><strong>Note:</strong></p> <ul> <li>A Sudoku board (partially filled) could be valid but is not necessarily solvable.</li> <li>Only the filled cells need to be validated according to the mentioned rules.</li> </ul> <p> </p> <p><strong>Example 1:</strong></p> <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png" style="height:250px; width:250px" /> <pre> <strong>Input:</strong> 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"]] <strong>Output:</strong> true </pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> board = [["8","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"]] <strong>Output:</strong> false <strong>Explanation:</strong> Same as Example 1, except with the <strong>5</strong> in the top left corner being modified to <strong>8</strong>. Since there are two 8's in the top left 3x3 sub-box, it is invalid. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>board.length == 9</code></li> <li><code>board[i].length == 9</code></li> <li><code>board[i][j]</code> is a digit <code>1-9</code> or <code>'.'</code>.</li> </ul>