A binary search tree (BST) is a binary tree where each node has a key and meet the following requirements

How to check if a binary tree is a binary search tree

Given a <code>binary tree</code>, check is this a <code>binary search tree</code>
<blockquote>
Binary search tree is defined as:
<ul>
<li><code>left</code> subtree of a node contains only nodes with values <code>less</code> than the <code>parent</code> value</li>
<li><code>right</code> subtree of a node contains only nodes with values <code>greater</code> than the <code>parent</code>value</li>
<li><code>left and right</code> subtrees must be <code>binary search trees</code></li>
</ul>
</blockquote>
 
<code>Example 1</code>
<pre><code>Input: 4 
 / \ 
 3 5 

Output: true
</code></pre>
<code>Example 2</code>
<pre><code>Input: 4 
 / \ 
 3 2 

Output: false
</code></pre>

80: Validate binary search tree