230 二叉搜索树中第 K 小的元素

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xvijdh/

cost: 5m47s

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        return this->kthSmallest_(root, k);
    }

private:
    int kthSmallest_(TreeNode* root, int& k) {
        if (root == nullptr) {
            return 0;
        }
        int r = this->kthSmallest_(root->left, k);
        if (k == 0) {
            return r;
        }
        if (k > 0) {
            k -= 1;
        }
        if (k == 0) {
            return root->val;
        }
        return this->kthSmallest_(root->right, k);
    }
};