Title

the problem link

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.

注意题目意思，root节点的值是最小的，所以最小值直接就是根节点的值了

Solution

//cpp:
/**
 * 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 minn=0;
    long ans=LONG_MAX;
    void dfs(TreeNode* t){
        if(!t)
            return;
        if(minn<t->val&&t->val<ans)
            ans=t->val;
        dfs(t->left);
        dfs(t->right);
    }
    int findSecondMinimumValue(TreeNode* root) {
        if(!root)
            return -1;
        minn=root->val;
        dfs(root);
        return ans<LONG_MAX?ans:-1;
        
    }
};