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+<h2>98. Validate Binary Search Tree</h2><h3>Medium</h3><hr><div><p>Given the <code>root</code> of a binary tree, <em>determine if it is a valid binary search tree (BST)</em>.</p>
+
+<p>A <strong>valid BST</strong> is defined as follows:</p>
+
+<ul>
+	<li>The left subtree of a node contains only nodes with keys <strong>less than</strong> the node's key.</li>
+	<li>The right subtree of a node contains only nodes with keys <strong>greater than</strong> the node's key.</li>
+	<li>Both the left and right subtrees must also be binary search trees.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg" style="width: 302px; height: 182px;">
+<pre><strong>Input:</strong> root = [2,1,3]
+<strong>Output:</strong> true
+</pre>
+
+<p><strong>Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg" style="width: 422px; height: 292px;">
+<pre><strong>Input:</strong> root = [5,1,4,null,null,3,6]
+<strong>Output:</strong> false
+<strong>Explanation:</strong> The root node's value is 5 but its right child's value is 4.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
+	<li><code>-2<sup>31</sup> &lt;= Node.val &lt;= 2<sup>31</sup> - 1</code></li>
+</ul>
+</div>