<h2><a href="https://leetcode.com/problems/valid-sudoku/">36. Valid Sudoku</a></h2><h3>Medium</h3><hr><div><p>Determine if a&nbsp;<code>9 x 9</code> Sudoku board&nbsp;is valid.&nbsp;Only the filled cells need to be validated&nbsp;<strong>according to the following rules</strong>:</p>

<ol>
	<li>Each row&nbsp;must contain the&nbsp;digits&nbsp;<code>1-9</code> without repetition.</li>
	<li>Each column must contain the digits&nbsp;<code>1-9</code>&nbsp;without repetition.</li>
	<li>Each of the nine&nbsp;<code>3 x 3</code> sub-boxes of the grid must contain the digits&nbsp;<code>1-9</code>&nbsp;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&nbsp;rules.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">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 class="example">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>&nbsp;</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>
</div>