AcWing 70. 二叉搜索树的第k个结点

/**
 * 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;
    }
    //dfs的参数k是一个引用
    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);
    }

};