0230. Kth Smallest Element in a BST.md

/**
 * 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) {
        int res = 0;
        int count = 0;

        function<bool(TreeNode*)> dfs = [&](TreeNode* node) -> bool {
            if(!node) return false;
            if(dfs(node->left)) return true;

            count++;
            if(count == k) {
                res = node->val;
                return true;
            }

            return dfs(node->right);
        };

        dfs(root);
        return res;
    }
};