https://leetcode.com/problems/second-minimum-node-in-a-binary-tree/description/

/**
 * 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:
    int findSecondMinimumValue(TreeNode* root) {
        set<int>res;
        queue<TreeNode*>nodes;
        nodes.push(root);
        while(!nodes.empty()){
            TreeNode* node = nodes.front();
            nodes.pop();
            res.insert(node->val);
            if(node->left) nodes.push(node->left);
            if(node->right) nodes.push(node->right);
        }
        set<int>::iterator it = res.begin();
        if(res.size()>1) return *(++it);
        else return -1;
    }
};