public class Solution
extends Object
671 - Second Minimum Node In a Binary Tree\.
Easy
Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly `two` or `zero` 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 `root.val = min(root.left.val, root.right.val)` always holds.
Given such a binary tree, you need to output the **second minimum** value in the set made of all the nodes' value in the whole tree.
If no such second minimum value exists, output -1 instead.
**Example 1:**
**Input:** root = [2,2,5,null,null,5,7]
**Output:** 5
**Explanation:** The smallest value is 2, the second smallest value is 5.
**Example 2:**
**Input:** root = [2,2,2]
**Output:** -1
**Explanation:** The smallest value is 2, but there isn't any second smallest value.
**Constraints:**
* The number of nodes in the tree is in the range `[1, 25]`.
* 1 <= Node.val <= 231 - 1
* `root.val == min(root.left.val, root.right.val)` for each internal node of the tree.