leetcode 671. 二叉树中第二小的节点(C++)

class Solution {
    int first;
public:
    int findSecondMinimumValue(TreeNode* root) {         
        first=root->val;
        return find(root);
    }
    int find(TreeNode* root){
        if(!root) return -1;
        if(root->val!=first) return root->val;
        int left=find(root->left);
        int right=find(root->right);
        
        if(left==-1) return right;
        if(right==-1) return left;
        return min(left,right);
    }
};