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.

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: 
    2
   /   2   5
     /     5   7

Output: 5
Explanation: The smallest value is 2, the second smallest value is 5.

Example 2:

Input: 
    2
   /   2   2

Output: -1
Explanation: The smallest value is 2, but there isn‘t any second smallest value.

求二叉樹中的第二小的節點：首先遍歷二叉樹後排序。然後在數組中找出第二小的元素即可。如果不存在第二小的元素，則返回-1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> res;
    
 
int findSecondMinimumValue(TreeNode* root) {
        inorder(root);
        sort(res.begin(), res.end());
        int smaller = -1;
        for (int i = 0; i != res.size() - 1; i++) {
            if (res[i + 1] - res[i] > 0) {
                smaller = res[i+1];
                break;
            }
        }
        return smaller;
    }
    vector<int> inorder(TreeNode* root) {
        if (root == nullptr)
            return res;
        inorder(root->left);
        res.push_back(root->val);
        inorder(root->right);
        return res;
    }
};
// 3 ms

[LeetCode] Second Minimum Node In a Binary Tree