LeetCode 230. Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

Just read the question carefully, then, there is no problem. BST.....

void inorder(TreeNode* root, int &res, int &idx,int k){
    if (root==NULL || idx>=k) return;
    inorder(root->left,res,idx,k);
    idx++;
    if (idx ==k ) res = root->val;
    inorder(root->right,res,idx,k);
}

int kthSmallest(TreeNode* root, int k) {
    int res;
    int idx=0;
    inorder(root,res,idx,k);
    return res;
}