Kth Smallest Element in a BST

/**
 * 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:
    bool inorder(TreeNode *root, int k, int &d, int &res) {
        if (!root) return false;
        if (inorder(root->left, k, d, res)) {
            return true;
        }
        d++;
        if (d == k) {
            res = root->val;
            return true;
        } 
        if (inorder(root->right, k, d, res)) {
            return true;
        }
        return false;
    }
    int kthSmallest(TreeNode* root, int k) {
        int res;
        int d = 0;
        inorder(root, k, d, res);
        return res;
    }
};