C++ 代码

class Solution {
public:
    TreeNode* ans;
    TreeNode* kthNode(TreeNode* root, int k) {
        dfs(root, k);
        return ans;
    }
    void dfs(TreeNode* root, int &k) {
        if (!root) return;
        dfs(root->left, k);
        k --;
        if (k == 0) ans = root;
        dfs(root->right, k);
    }
};