算法1

(中序遍历) $O(n)$

C++ 代码

/**
 * 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:
    void dfs(vector<int>& v,int k,TreeNode* root){
        if (v.size() == k){
          res = v[v.size()-1];
          return;  
        } 
        if (!root) return;
        if(root){
            dfs(v,k,root->left);
            // cout<<root->val<<endl;
            v.push_back(root->val);
            dfs(v,k,root->right);
        }   
    }
    int res = 0;
    TreeNode* kthNode(TreeNode* root, int k) {

        vector<int> v;
        dfs(v,k,root);
        // res = v[v.size()-1];
        TreeNode *p = new TreeNode(res);
        return p;
    }
};