binary-tree-level-order-traversal-ii.md

<p>Given a binary tree, return the <i>bottom-up level order</i> traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).</p>

<p>
For example:<br />
Given binary tree <code>[3,9,20,null,null,15,7]</code>,<br />
<pre>
    3
   / \
  9  20
    /  \
   15   7
</pre>
</p>
<p>
return its bottom-up level order traversal as:<br />
<pre>
[
  [15,7],
  [9,20],
  [3]
]
</pre>
</p>