<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's value is the smaller value among its two sub-nodes. More formally, the property <code>root.val = min(root.left.val, root.right.val)</code> 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' value in the whole tree.</p> <p>If no such second minimum value exists, output -1 instead.</p> <p> </p> <p> </p> <p><strong>Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2020/10/15/smbt1.jpg" style="width: 431px; height: 302px;" /> <pre> <strong>Input:</strong> root = [2,2,5,null,null,5,7] <strong>Output:</strong> 5 <strong>Explanation:</strong> The smallest value is 2, the second smallest value is 5. </pre> <p><strong>Example 2:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2020/10/15/smbt2.jpg" style="width: 321px; height: 182px;" /> <pre> <strong>Input:</strong> root = [2,2,2] <strong>Output:</strong> -1 <strong>Explanation:</strong> The smallest value is 2, but there isn't any second smallest value. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li>The number of nodes in the tree is in the range <code>[1, 25]</code>.</li> <li><code>1 <= Node.val <= 2<sup>31</sup> - 1</code></li> <li><code>root.val == min(root.left.val, root.right.val)</code> for each internal node of the tree.</li> </ul>