LeetCode 230. Kth Smallest Element in a BST

class Solution {
public:
int rank;
int result;

void help(TreeNode* root, int k){
if(!root) return;

help(root->left, k);
if(++rank == k){
result = root->val;
return;
}
help(root->right, k);
}
int kthSmallest(TreeNode* root, int k) {
rank = 0;
help(root, k);
return result;
}
};