Given a binary tree, return binary tree size. Size of the binary tree is number of nodes in the tree.

How to count a binary tree size

Given a <code>binary tree</code>, return binary tree <code>size</code>
<blockquote>
Note : Size of the <code>binary tree</code> is number of nodes in the tree
</blockquote>
 
<code>Example 1</code>
<pre><code>Input: 1 
 / \
 2 3

Output: 3
</code></pre>
<code>Example 2</code>
<pre><code>Input: 1 
 \ 
 3 

Output: 2
</code></pre>

73: Binary tree size