Solution (leetcode-in-java 1.19 API)

java.lang.Object
- g0601_0700.s0671_second_minimum_node_in_a_binary_tree.Solution

```
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.

- Constructor Summary
  
  Constructors
  Constructor Description
  
  Solution()
- Method Summary
  
  All Methods Instance Methods Concrete Methods
  Modifier and Type Method Description
  
  int findSecondMinimumValue(TreeNode root)
  - Methods inherited from class java.lang.Object
    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail
- Solution
```
public Solution()
```

Method Detail

findSecondMinimumValue

public int findSecondMinimumValue(TreeNode root)