/**
 * 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:
    TreeNode*  ans;
    TreeNode* kthNode(TreeNode* root, int k) {
        dfs(root,k);
        return ans;
    }
    //k传递是引用以确保每次改变
    void dfs(TreeNode* root,int &k){
        if(!root) return;
        dfs(root->left,k);
        k--;
        if(!k)  ans = root;
        if(k > 0) dfs(root->right,k);
    }
};