second-minimum-node-in-a-binary-tree.md

<p>Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly <code>two</code> or <code>zero</code> sub-node. If the node has two sub-nodes, then this node&#39;s value is the smaller value among its two sub-nodes. More formally, the property&nbsp;<code>root.val = min(root.left.val, root.right.val)</code>&nbsp;always holds.</p>

<p>Given such a binary tree, you need to output the <b>second minimum</b> value in the set made of all the nodes&#39; value in the whole tree.</p>

<p>If no such second minimum value exists, output -1 instead.</p>

<p><b>Example 1:</b></p>

<pre>
<b>Input:</b> 
    2
   / \
  2   5
     / \
    5   7

<b>Output:</b> 5
<b>Explanation:</b> The smallest value is 2, the second smallest value is 5.
</pre>

<p>&nbsp;</p>

<p><b>Example 2:</b></p>

<pre>
<b>Input:</b> 
    2
   / \
  2   2

<b>Output:</b> -1
<b>Explanation:</b> The smallest value is 2, but there isn&#39;t any second smallest value.
</pre>

<p>&nbsp;</p>