binary-tree-preorder-traversal.md

<p>Given a binary tree, return the <em>preorder</em> traversal of its nodes&#39; values.</p>

<p><strong>Example:</strong></p>

<pre>
<strong>Input:</strong>&nbsp;<code>[1,null,2,3]</code>
   1
    \
     2
    /
   3

<strong>Output:</strong>&nbsp;<code>[1,2,3]</code>
</pre>

<p><strong>Follow up:</strong> Recursive solution is trivial, could you do it iteratively?</p>