230. Kth Smallest Element in a BST

LeetCode

We know that the in-order traversal sequence of a binary search tree is sorted, thus we can use it solve the problem.

/**
 * 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 {
private:
    void dfs(TreeNode* root, int& k, int& res) {
        if (root == nullptr) {
            return;
        }
        dfs(root->left, k, res);
        k--;
        if (k == 0) {
            res = root->val;
            return;
        }
        dfs(root->right, k, res);
    }
public:
    int kthSmallest(TreeNode* root, int k) {
        int res = -1;
        dfs(root, k, res);
        return res;
    }
};