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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;
}
};