sum-of-left-leaves.md

<p>Find the sum of all left leaves in a given binary tree.</p>

<p><b>Example:</b>
<pre>
    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values <b>9</b> and <b>15</b> respectively. Return <b>24</b>.
</pre>
</p>