LeetCode 671. Second Minimum Node In a Binary Tree

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.

/**
 * 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 { //利用set来过滤掉重复的数，先序遍历来将所有的节点数值收入set中
public:
    void pre_order(TreeNode* root, set<int> &porder){
        if(root)
            porder.insert(root->val);
        else
            return ;
        pre_order(root->left, porder);
        pre_order(root->right, porder);
    }
    int findSecondMinimumValue(TreeNode* root) {
        set<int> porder;
        pre_order(root, porder);
        if(porder.size()<2)
            return -1;
        else
            return *(++porder.begin());
    }
};